What makes a cheater a cheater is the person's ability to avoid being caught up at least for a while.
She can always create another email address.
You may look for signs such as emotional/sexual intimacy with you (if she seems distant and cold) If she seems to be trying to hide in order to make phone calls or when she's receiving ph calls, try to look at the bank statements (if you share a mutual account) or the phone record. There is always the option of hiring a PI to do the work for you.
Try to talk to her and see where she wants the marriage to go in her honest estimation.