I hate to be snoop but you should:

1. Go through her email (all accounts)

2. Check her incoming and outgoing phone calls and try to recognize the numbers

3. Check any text messages on her phone

4. Check all IM's

5. Ask where she is going out that night and drive to parking lot to see if her car is there.

6. Check her mileage odometer when she goes out.

7. Test her with "what would you do if you found out I was cheating on you?" Gauge her reaction. If it is non-chalant then red flag.