The answer is "yes".
Basically, all brake fluid is synthetic. The glycols in brake fluid are manufactured chemicals. Some brands put "synthetic" on the container in hopes you will somehow associate their brand with synthetic motor oil and become impressed enough to buy their brand. There's no way to tell what was written on the container, and it doesn't matter anyway.
All DOT 3, 4, and 5.1 fluids are glycol-based, mixable and chemically compatible. Personally, I would flush and fill with a quality DOT4 fluid and worry about something more important, like toenail fungus. (5.1 fluids can have super-high boiling points, but generally aren't good choices for street vehicles.)
If that answers the question, great.
Or was your question more along the lines of "How can I tell silicone based DOT 5 fluid from glycol based DOT 5.1/4/3 fluid?" (Both are synthetic chemicals.)
One test might be to add a few drops of water to a sample and mix. DOT 3/4/5.1 should absorb the water, and DOT 5 will not.
I can say that if your kit car's braking components were originally designed for a street vehicle, they were designed to use DOT 3 or 4 fluid. What some misguided past mechanic would have done is difficult to say -- you definitely do NOT want to mix DOT5 with the others.
Personally, I'd open the top of the master cylinder and take a sniff. Your nose is a pretty decent chemical analyzer, and if it has turned sorta brown like normal old brake fluid and if it smells sorta tangy like normal old brake fluid, then that's what it is.
Glycol-based DOT 3. 4. 5.1 fluid is required to be clear or amber (ATE had to take their blue fluid off the market a few years ago) and of course it turns brown as it absorbs moisture, as we all know.
Silicone-based DOT 5 fluid is supposed to be purple. It doesn't absorb water, but if the system is contaminated rust can form and cause discoloration. It also tends to turn amber or brown as it ages, so color isn't definitive.
