There is the 3 proof offs that might be asked by an officer, if you are coming in on the 30 days waiver. Proof of accommodation, proof of onward travel and proof of sufficient funds (20k thb in cash or equivalent in other currency). But totally up to the officer wether to ask for or not.
PP is notorious for this, Vientiane or Savannakhet always better choices but I believe they became much stricter with checking wether or not you have the right visa for your stay in TH
Google : onward ticket. This caters for the matter that you might be asked for a ticket out of Thailand, either by the airline or immigration. The first more likely, asked at check in.