8 3/4 that I will start to prep for use. How do I check for straight. The rear is fully assembled less brakes

Depends what you mean by straight. Also depends on what tools you have or are willing to buy.

I use two carpenter squares and a tape measure , have some one hold one of the squares against the axle flange, no brake drum or rotor on the axles and measure out around 15 inches from the center of the axles on both sides to the front side and see what you get, then do the same towards the rear and then if you can straight up and down. The stock wheel tubs limit how far you can measure straight up, what ever you can get use the same length measuring straight down
I like to see less than 1/8 inch variance front to back with the front being narrower for toe in, no variance straight up compared to straight down
Please let us know what you do no mater what method you choose

Unbolt the axles, slide them in and out. Check for square, having to pull them one way or the other (likely back if its bent) to get them to slide in, and going in and out smoothly.

I've seen some hacked up narrow jobs with the ends on stupid crooked and the axles wouldn't even go in all the way.

2 jack stands ... roll it over with a dial indicators at the ends. that gets the housing checked.

THIS ^^^

Being a cabinet maker, I thought about a straight edge on each flange and measure the diagonals for and aft of the housing in general. That work?

Absolutely
I measured off the ends of the squares instead of across and over the housing, I like and use the KISS principles

Great idea but worthless info until you have a specification to go against.

Use a 4’ long straight edge and it has 1/8” toe in versus a 1’ long straight edge and it has 1/8” in toe in.
That’s two different values in degrees of toe.

What’s the spec?

Now if you get zero in each case - you may be on to something. And that is assuming the ends were welded on square to the tubes.

In the OPs case, I would go with McAllister’s suggestion.
Starting from scratch - use an arbor and pucks.

As I said I look for no more than 1/8 inch toe in on the front side 15 inches out from the axle centers, .060 narrower on the front and .060 wider on the back side, no difference between the top and bottom measurements. This is to check if the housing or axles are bent at all, I do rotate the axles 90 degrees to make sure if the width differences in the axles or the housing
I bent the 1964 B body narrowed 8 3/4 housing in my old Duster bad enough that the right side axle wouldn't slide in and out without using a block of wood and a big hammer to drive them in
Never again

The simplest and quickest way I have used for preliminary checking for straight is put a set of tires or even just rims on, while it is sitting on floor, rotate the housing around and you will see wheels move in and out as it is rotated. I have seen supposedly STRAIGHT housings that were done with the solid bar narrowing kits when welding still show quite a bit of wheels moving in and out as housing is rotated. Again, quick and simple, but definitely lets you know how straight..... or not!

Cab, where did you find the spec. of 1/8” using a 15” dimension?

Roundy round racers years ago

exactly .... i have a jig made just for this. you chain the rear down over the rims and roll it over 1 time . we use a jack to re-square them. rims must be true..... i used this on circle track rears for years. we got bumped all the time and it was the fastest sure fire way to check and repair them all at 1 time.... sliding the axels out will let you know if its bad enough too ..