If you put the loco on the track (DCC powered) AND it makes a 'buzzing' noise, then it does not have a decoder fitted. - It will run if you select address '0', but this is not kind to the motor for any length of time (it can get hot), so once tested to see if moves take it off the track, please.
Crude, but effective..
Of course, this only prove the loco does not have a decoder, if it makes a noise (switched on, and not faulty).