I think general knowledge of customs (that he would have learned in his pre-Superman days) and regional dialects/languages could give him away. Turns of phrase could also show that he'd been here longer than he claimed to be. While he could have picked them up in a short time, his complete comfort in suing the phrases/customs could be the dead giveaway. Discussing regional foods could clue people in too - if he was only "here" for "a month or two" but talks about eating food from every corner of the world (while also being the most in-demand person on the planet), people could easily put 2+2 together - "Man, he's mentioned like 384 different dishes from different countries but he's had time to eat...what? Two dozen meals?"
