Most airlines have their own distinct registration series.
Deltas N numbers usually end in DL, for example.
You could spot the ex-Northwest DC-9s that had originally belonged to Republic because their regs ended in NC, or North Central. Northwest had DC-9s that came from a number of different carriers' fleets.
The 717s have regs ending in AT, the first operator of these aircraft, as do the 737s from that carrier that Southwest kept.
United aircraft have regs usually ending in UA.
With any airline, it's usually easy to spot the prior operator of an aircraft that saw service before joining its current fleet by the alpha numeric reg series.
This is nothing exclusive to WOW but is rather something spotters have been hip to for many years.