The short answer is that there is no official channel that will confirm this for you, and pursuing it through back channels is unlikely to get you what you are actually looking for.
Corrections officers and staff are not going to discuss the personal relationships of inmates with people on the outside. That falls squarely under the privacy protections that cover inmate conduct inside the facility, and any staff member who shared that kind of information would be breaking the rules at minimum and potentially their employment at worst. Another inmate passing along information is possible but unreliable and comes with its own complications.
Here is the more honest framing of the situation. Prison relationships between inmates happen, and they happen for reasons that make sense in context. Incarceration is isolating, emotionally demanding, and the connections formed inside are real even when they are complicated. That does not make it easier to sit with from the outside.
What you can actually do is talk to your girlfriend directly. If the suspicion is strong enough that you are looking for confirmation through guards and other inmates, it is strong enough to put on the table in a direct conversation. How she responds to that question will tell you more than any third party source could.
The harder question underneath this one is what you plan to do with the information once you have it, and that is worth thinking through before you go further. If she confirms it, what changes? If she denies it, will you believe her? Those questions matter as much as the answer itself.
Trust what you already sense. And then decide what you want to do about it.