If you don't meet that often, you may be able to tell next time. However if you are in a position to have a random conversation with her, and it seems appropriate to be talking about nothing in particular, bring up something like weird food combos or the like. Then the conversation would likely go as follows: you: "You know what I just discovered? I know it sounds strange, but Doritos and Oreo's at the same time tastes awesome." her: "OMG! I know what you mean, since I got pregnant, I've been into this pickles and hot sauce kick! you should try it!" If she's pregnant, she will mention it in that context. If she's just gained belly fat, or doesn't want you to know, she might still talk about food, but not pregnancy.