Any fool proof ways to determine if brake fluid is dot 3 or 4
Versus dot 5 silicone ?

Good question! Not to be asmartxxx, they probably taste differently

Its a car I did for a client about 5 or 6 years ago.

And I cant remember if I used dot 5 or not.

I am now 2000 miles away from said client and I simply dont remember what i put in it .
And his car is low on fluid and doesnt want to mix fluid types if possible

Pretty sure Dot 5 is purple in color.

dot 5 is purple.
dot 3 & 4 are light amber[ish] in color.
i wouldn't put 5 in if you suspect it truly isn't purple color.
as to 3 & 4, i would just add some 4.
if a true color can NOT be determined, to be on the safe side, i would drain, flush with alcohol, then fill with your choice of fluid.
also, don't confuse dot 5 with dot 5.1. 5 is silicon [?] based, while 5.1 is not. also, i do not know what color 5.1 is, although i suspect it may be close to the color of 3 & 4.

The purple can fade over time. Not sure about 5 years, but I did flush once after about 15 years and it was not purple any longer.

I say put some fresh DOT 3 in a clear glass and put a sample of this fluid in it. If it mixes, should be same DOT rating. If it is DOT5, it will separate.

Put a few drops on a freshly painted surface, if the paint curdles, it isn't DOT 5 silicone.

Thanks all

He said fluid in it was a clearish color. And was wondering if the color faded over time. But im pretty sure dot 3 does not

Owner went back thru invoices and found dot5 billed...which i was pretty sure what i put in it.

He is going to flush and refill it anyway...simply from age. But going back with dot5

Gonna have to remember the mixing method test for future reference

Standard fluid will mix with water. Silicone will not.
Doug