You're treading a really thin HR line. If you have concerns about her performance at work, I'd address that entirely independent of the assumed cell phone usage.
That said, if you have a written network policy at work, chances are you have the right to monitor all traffic on your network. This will only work for her on wifi, but chances are she's using that instead of wasting battery and data usage on a 3g/4g connection. You'd need to find her MAC address to have complete certainty. You could get that with some effort, but I really don't think the effort is worth it.
If you have concerns with an employee's productivity, address that. Don't make allegations. It's a bad look.