Your originating airline will be the one that checks to make sure you are leaving Thailand before your entry permission ends. They may not let you board if you can not document that you are leaving Thailand within 30 days of your arrival. And the only true proof of this is a departing airline ticket to a neighboring or home country. Train tickets and bus tickets are not legal proof, but maybe accepted (then again may not be) by the agent.
Thai Immigration rarely checks this aspect, but they do have the right to, and I have personally witnessed them asking people to prove such. So not having proof is a double crap shoot.
Best bet is to buy an airline ticket to a neighboring country that leaves before your 30 days are up. Either a dirt cheap throw away airfare (Air Asia has a sale going on right now) or an expensive fully refundable airfare. Then you can leave your flight home as you originally planned.
You can also apply for a tourist visa in advance, which would give you 60 days entry and possibility for a 30 day extension. While this still requires proof of exit, airlines tend to overlook the outbound rule when boarding (but not 100% guaranteed, as I had to talk my way on once with a Non-Imm visa for a longer stay). You have to weigh costs of the Visa vs cost for a cheap airfare.