ATF has a very distinctive odor, you should be able to recognize it pretty easily. if you aren't familiar with it, just check the transmission dipstick on your car. Take a wiff of that, that odor is very distinctive, hard to miss even if you have a bad sense of smell.
Also, this isn't a very demanding application, it's not like its a tractor digging or moving dirt all day long; it's not going to make a huge difference what you use. There is not a lot of danger in mixing common oils like this, the additive chemistry may not be a perfect match, but there shouldn't be a problem with negative reactions occurring if you added ATF into a system originally filled with anti-wear hydraulic oil, or vice versa. The important thing noted in a message above is DO NOT USE BRAKE FLUID!. Brake fluid is a particular type of fluid that if you added it into your leveling system it would damage the seals and you'd have to have the entire system rebuilt.