as said,,when idling you may smell a little derv fume and the tips will be sooty..but only real way will be to get it in the air and take under trays off,either its been removed or gutted,,either way you will be able to tell,unless the dpf box has been welded up very very neatly.but i doubt it looks as neat as when left factoryalso if you have not had any regen problems or even a regen

how can you tell if a dpf has been removed