For everything to go as planned, you need a few things that absolutely MUST work.

1.) A "known good" carb (and fuel pump)
2.) A "known good" ignition
3.) Stable and sufficent voltage, at least a good battery, if not a "known good" alternator and regulator as well.

Mixture has to be close enough to run for 20 minutes without overheating
Ignition must work and timing has to be quickly adjustable , also no overheating.

SO.

How do you KNOW if brand new untested carb and ignition parts will actually work ?

I don't think you can know that.

I have an OLD orange ignition box that worked 25 years ago. But I am thinking about firing the engine with a points distributor / coil, one that has been
checked on an old distributor machine and has a known advance curve. If the points gap is set, it should fire the engine, right ?

On the carb, if I put on a brand new Edelbrock/Carter and go through it first to make sure there is no debris in it, it should fire the engine, right ?

What else am I missing ?

As far as the carburetor, make sure to fill the float bowls with gasoline before trying to fire the engine. You want the engine to fire immediately so as to get oil flowing quickly.
It kills me to watch some of the car shows when they try firing an engine without filling the carburetor first. They crank and crank and crank waiting on the fuel pump to start getting fuel into the carburetor. It makes me cringe thinking about the wear happening on the lifters before the engine fires.

Make sure you have a good cooling system and there are no air pockets. If you have access to an Air Lift coolant filling system use it because it will also show coolant leaks. If you are breaking in a new flat cam follow the cam grinders recommendations to the letter for success. The card that came with my Howards cam had a procedure that is a little different than I'm used to. It was a few 10 minute intervals instead of the usual 20 minute straight deal which I prefer because you time to check things as you go.

Gus

We set ignition timing during build. What initial do you want? rotate engine to that point before TDC and rotate distributor so the points just open or reductor in line with coil. This will be close enough. Test fire distributor for spark before installation. On the run stand or cars system before hand. On day of firing, Use a test distributor to test ignition system again just be for cranking. Good spark you are ready. We use an electric pump through mechanical. After initial run, you can turn it off to ensure pump is working sufficiently.

I set my carb up and put it on a friends engine and dialed it in so that was a non issue . set your timing and have a friend with you for an extra set of eyes.

On a big block spin over to tdc and the distributor gear will be able to be moved to parallel with the crank(slot will be perfectly straight across looking down in the hole. This will put the rotor right on tdc when you drop in the distributor. If you are 180 out, the slot will be turned ever so slightly. Once you see both ways for the first time it’s obvious from then on.

I prime the oil pump just before starting and spin the motor over with the plugs out on the starter then set the crank timing marks at 12 to 18 BTDC and install the distributor so it is firing #1 aligning the magnetic pickup with the distributor cap off and use a known good carb and prime it with some fresh gas, I set the total advance as soon as possible to 34 BTDC on BB, 426 Hemi and 32 on SB with the motor running above the full advance RPM
I do try and vary the RPM from 1500 RPM up to 2500 RPM revving it several times about every 2 to 3 minutes to sling more oil on the cam and load the rings a tiny bit to help seat them

I like to have the car ready to drive then when the cam break in is over and all looks good and no leaks I go drive it right then to help seal the rings. Never let to cool off just go drive it.

disable the starter & have a helper hold the ign to "start" with a spare dist plugged in & spin the dist by hand & confirm that the wiring is correct & that the ECU is firing at the plugs. do this with the key in "run" also. turn the crank to #6 & #8 to preoil the heads ( BB) or 20 deg ATDC #6 compression and 90 deg (1/4 turn) BTDC #1 compression for SB. Have initial at 15-20 BTDC to fire it (can depend on cam). not sure what total to set it at when it fires. UTG says ign boxes (I think he was referring to orange) fire later than what the timing marks say for initial (not sure if that is correct or not). #1 as you know you is want it to fire within several revolutions of cranking.

You can, but have to be willing to jump through flaming hoops of fire and so on.

an entire ignition can be wired up and laid on a bench, all the way to 8 spark plugs, the distributor can be spun by hand or gently placed in a vise
and spun with a drill motor, spark can then be observed on the bench for every cylinder. This is good for puzzling situations where everything else has failed.
If the spark looks like weak orange junk, that's exactly what it is and it needs to be fixed.

Alternately, before the last step of priming, with the distributor out of the engine, you can take a spare plug wire, attach a spark plug, run that to your
coil tower out of the coil, turn the ignition on, spin the distributor by hand and observe the spark. This ensures the entire primary circuit of the ignition functions, and takes the secondary side of the ignition on faith (this is how I do it when reasonably confident the cap rotor and wires are ok).

You'll encounter many who will say this kind of thing is ridiculous, they're usually the same folks who crank the engine for an hour, eventually find the error and get the engine running, then months later blame the cam when it goes flat.

As far as the carb goes....with the carb suspended/not sitting on a bench, fill the bowls manually through the bowl vents, hold the butterflies wide open and blow 100psi or so compressed air through the barrels with a basic air nozzle. Do this for a fraction of a second and observe. You should be able to see fuel being pulled through the booster venturi. Assuming you can, the carb will generally work well enough to at least run on the main metering circuit. It may not "run well" but with the throttle open sufficiently it can get you through break in. (Put out your Marlboro Red when doing this. Smoking is bad.) Bolt the carb on the engine after that.

Many might be surprised to hear you can start an engine with no carburetor at all, nothing but a squirt bottle of fuel and the right touch.
The problem with this is, rpm is not controllable. It is strictly a junkyard/salvage engine "must hear it run" kind of a test only good for a second or two.

Many might be surprised that 20+ minutes straight is totally not necessary AS LONG AS the engine fires right up easily.
Instead, you can do 3 to 5 minutes at a time, avoid getting anything hot, and also establish the desired wear pattern on an antiquated flat tappet
setup with a whole lot less stress, and plenty of opportunities to take care of any issues that come up.