Normal inspection as per any stringed instrument.
The state of the fingerboard (surface condition, radius etc), neck (truss rod) and nut would be my number 1 comment. This is especially important if the neck has been de-fretted. Look for any ridges on the fret-lines and/or excessive gouging along under where the strings lie. The nut height is important if you want the lower action typical of a fretless bass.
When trying the bass, first play it without amplification. This will help you discern if the notes sustain well, which helps a fretless bass sparkle.
Personally, rolled edges to the fingerboard are a “nice to have”.
Remember that a lot of the fretless sound comes from player skill and articulation. Good luck in your search.