Taste it. If it tastes like dish detergent, ATF. Taste like a tarnished copper penny that spent 10 years on asphault, oil. If it tastes like dirty socks, or gym shorts, coolant. Maybe it tastes like ammonia, dog pee.

But if you don't think it's a smart idea to get down on all fours and start liking an unknown puddle, I'd guess ATF.
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