Do they test for dmt at airports?

Of course they do.  They have sensors in the walls that detect dmt down to ppt.  When a person who has any traces of it on their clothes, hair or skin walks passed it, the sensor detects it and triggers an alarm.  That's what the little side rooms are for.  People who have been "detected".  Then, "they" mark your passport and "they" pull you aside and search you and your belongings every time to fly. 

passengers are not tested for drugs on air flights.

why would they? and would you have all the passengers wait a few days for the test results to come back?

ps: DMT has many meanings, which one is this?

    N,N-Dimethyltryptamine, a naturally occurring psychedelic compound

    Desoxymethyltestosterone, a designer anabolic steroid        

    Dimethyl terephthalate, a polyester precursor

    DMT1, a transporter involved in human iron metabolism       

    Dimethylglycine N-methyltransferase, an enzyme

Nope.  You cannot test for what does not exist.