To make this case more complicated: Say that said someone had two passports (of the same country) - only one passport associated with said visa. Could / should this someone maybe use the other passport for the 30 day exempt, first, and then fly out, and return on the other passport with the Thai visa? Or would changing passports in between raise more red flags then it would benefit the case?
So I guess, just not present the eVisa and see what kind of stamp is issued. If 30 day exemption, that visa should still be valid. If 60 days, the IO used up the single entry visa.