Is there a quick way to tell between the two?
I got my old car back after 17 years and can't remember which I had in it.

Also don't know if the two previous owners had switched to what ever?

Suck some out of the master cylinder and put in in a small clear jar. Add a few drops of water to it, if it's silicone the water will quickly seperate from the brake fluid. If it's dot 3 or dot 4 the water will be slowly absorbed into the fluid.

Thanks I thought it would be something like that

That beats my idea of seeing if it bubbled up the paint, lol.

It is alleged that silicone fluid is suppose to have a purple tint to it. I have in our 300 B Chrysler and it is purple. However would not wager that this always holds true.

I have seen advertised somewhere a product of test strips that you can sample your fluid by dunking them into it and this will tell you what fluid you have. Not to be confused with the strips that tell you percentage of water in you dot 3 and 4 fluids.

Don't they smell different?

Do they taste different?

Dot 5 is very purple. Another trick you can do is put the brake fluid in your car on something painted. If it eats the paint, it isn't DOT 5.

Another idea would be to dip a rubber o ring into the fluid. If it swells up it would be DOT 3/4.

Water test is best. The silicone starts purple but its color can change with age.
Doug

Thanks everyone
I'll try them all just to see what works best
I'm at work now i'll try this evening.