Where can I check someone's mobile phone text messages?

One way to do something like this is to have all calls (including text messages) forwarded to another (your) cell phone. This works best for just short periods of time and you have to be ready to reverse it quickly so that he doesnt find out. I would try doing this one night as he is sleeping and make sure that you UNFORWARD the calls by morning time.

Another thing you might try to do and this is really sneaky and probably won't work for very long : switch the SIM card from his phone with a new, unused one. Put his SIM card into your phone and you will get any calls or texts that are sent to him. How long this works depends on how long it takes for him to notice that there is something wrong with his phone and how long it takes for him to get it to a phone dealer for repair. The nice thing is that there is really no way to figure out what happened so you should never be suspected.