If your mom is dating behind your back, you should not be angry rather you should wait for her to come clean about it. You can ask her politely about it and get detailed on what's happening. You can comment on it but respect her decision always.
If you know she has a date tonight with him, say "Hey mom, why don't we catch a movie?" Eventually, you will crack the code and she will spill the beans about it. Don't worry! Just hang in there my dear!