You have nothing to gain by knowing, you wouldn't rub salt in a wound.
You desperately want to know and there is a big difference between wanting and needing to know. You are both involved in game playing here, the most tragic and hurtful kind of hunt the thimble. What happens if you find the thimble? Pop it back in the sewing basket and carry on as before?
What is it that you 'know'? What can you do with this 'knowledge'? Will this 'knowledge' provide you with a future, financial security and happiness or is it being held to inflict damage and destruction to make you feel better.
Least said and soonest mended, stay calm and focus on what is real - start sorting out your finances - open a savings account with a passbook so statements don't come through the door, concentrate on what YOU are GOING to do rather than what he MAYBE doing.
You are only going to drive yourself insane. Jealousy and possesiveness are two hugely negative emotions and sound the death knell for many relationships. Start dealing with the negatives
Remember it is not normal behaviour to snoop through a partners phone etc - it means there is a lack of trust there. Where did that lack of trust come from? Is it real or imagined?
Chin up GC - you are probably a lot stronger than you think