someone told me a sign was if you floor the gas and your car goes nowhere, or it takes a few seconds to accelerate after you floor the gas. Basically, I guess your rpm's shoot up, but your car slugs.
Or another one I heard was, you try to deliberately stall the car but you can't. For example, you put it in 1st and pop the clutch (no gas). If it doesn't stall, then something is wrong.