There are no "only" ways. There are MULTIPLE ways. Keylogger on her computer, GPS on her car, PI, voice-activated recorder hidden under the seat of her car, etc, etc.
But one easy way is to suddenly, right after she's been texting, ask to see her phone. Have her SHOW it to you. Say "You've been acting strange lately, and I can't help but feeling that there's something you're hiding from me. Show me your phone, so I'll know I'm totally off-base here. Please know that there is another way I can get the same information, but I thought I'd ask you first, to give you an opportunity to tell me the truth."
Her response should tell you all you need to know.
