1- Write down how often he texts you per day. You may discover a pattern. If he suddenly starts sending you many fewer texts per day, you should be concerned. He may be conserving his credit for another girl so you may want to check his messages. If you discover texts from more than one girl (not including his mother or sister) he is almost definitely cheating on you and most probably with multiple others.
2-If he buys you an overwhelming amount of flowers or chocolates, this is cause for serious concern.
3-As a last resort, you may want to hire a private detective. It is expensive but more often than not it is extremely necessary.
Checking his receipts may give you clues as to the gifts he is buying for others.
If he has a diary, find a quiet moment when he is not around and photocopy it. You may want to use this material when confronting him later.
Do not use weaponry or violence when confronting your boyfriend. This will lead to someone getting hurt.