Not always
I did 20k miles before removing the DPF
Easy way to tell is how black the tail pipes look. A DPF is very effective at trapping the soot and exhaust pipes will be very clean.
Also see if it smokes on the test drive.
Finally stick your head under the bonnet and look down by the bulkhead. If you see a big filter thing then the DPF is still in place....unless they have simply had the guts removed.....