How to tell what brake fluid I have...

OAK and WER, good links.
I think this is important enuff to summarize the relevant takeaways from those links:

DOT 3, 4, and 5.1 are glycol (alcohol) based fluids.
DOT 3 is obsolete, but still available. DOT 4 and the newest 5.1 offer increasing levels of heat tolerance.
Being alcohol based, they will absorb moisture and will turn darker over time as they absorb that moisture (a good indicator of freshness).
Alcohol based fluids should be replaced/flushed every 2 years.

DOT 5 is a silicone based fluid.
DOT 5 has the highest heat tolerance, but doesn't absorb moisture.
Silicone based fluid will demonstrate a more compressive 'squishy' feel in the master cylinder.

The two types of brake fluids are incompatable, do not intermix.
Any conversion of one type to the other requires complete teardown and cleaning of the entire hydraulic system to remove all traces of the previous type.

To test for fluid type:
Place a little water in a clear glass.
Add some of the suspect brake fluid.
Shake the fluids in the glass.
The alcohol based fluids will mix/disperse/dissolve in the water.
The silicone based fluid will separate from the water, showing a dividing line between the fluids.

Personally, I don't recommend the 'taste' test.

From the 'SteelSoldiers' military vehicles website:
BrakeFluids.jpg
 
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