Hi,
I am afraid that there is no one "test" that will enable you to get the answer (obviously he'll hide it). But there are million tiny things that you should detect / notice. He is late, you don't know where he is for several hours, you can't reach him on the phone, he pays less attention 2U, he keeps more silent than usual, has less sex than usual, changes the tone of his voice towards you (e.g: less polite), goes out of the house frequently, more phone calls (or text) than usual, spends more money than usual, buys more clothes than usual, takes more care to his apearance than usual, etc, etc.....
Best regards,