First of all: Ask her. If she says no, trust her and do not open the subject anymore. If she says no, but you do not believe her, or she doesn't seem trustworthy, then you can ask her to check her phone, computer, email, etc. If she does not want you to check her personal inboxes, and she does not give you a good enough reason for the rejection, then you should really worry about.
Do not get her phone from her hands using strength, ever. You can give her an ultimatum: tell me the truth or I am leaving for example.
Or you can check her phone when she is having a shower lets say. But the best way is to have a conversation about this issue, and tell her that being a lier is not going to solve anything. If you have to lie for something, why bother at all, because your marriage won't matter anymore. Remind her that you both have the option to leave, so no need for cheating or lies.
Anyway I hope that it is not true at all, and you 'll have a wonderful marriage. The best of luck :)