The best way to check the polarity of the balun is with some sort of a signal strength meter. You'd have the two antennas aimed at a signal source (something like an analog channel in the low-VHF band would be ideal, but some signal in that general range would work...maybe a VHF two-way radio transmitter nearby), with the cables cut to matching lengths and feeding a splitter/combiner. With the baluns installed one way, check the signal strength. Then, flip one of the baluns over, and see if the signal goes up, or down. You want the position that adds, giving the higher signal. A radio scanner with a signal strength meter would work. A friendly Ham radio operator (with test gear) is even better.
The meter on a Digital TV is not good, since it reads lots more than just signal strength.