Unfortunately the result is affected much more often than "in extremely rare instances". Search instability is a well-known phenomenon in alpha-beta search, and transposition tables are responsible for quite a bit of it. In chess this is particularly easy to trigger in situations where draw by repetition or the 50-move rule are involved.
Also, how can I test transposition tables to check that the code I added with regards to TT works fine?
One thing I've done is to perform each search with TT on and TT off and then check that the same result is being returned. The TT shouldn't affect the result (only in extremely rare instances) compared to a normal search - it should only affect the speed.