The fastest way to check is through this PPO benefits calculator, which gives a rough estimate in a couple of minutes.
The most accurate way is a quick call to your health insurer. Call the number on the back of your health card, ask for member services, and say you want your out-of-network outpatient mental health benefits. Then ask:
Why the allowed amount matters. Your plan doesn't reimburse a percent of what you actually paid. It reimburses a percent of what it decides a session should cost, called the "allowed amount". So if a session is $250, your allowed amount is $175, and the plan covers 70%, you get back 70% of $175 (about $122), not 70% of $250.