I can't really tell how likely a current or 
previous owner of a cel will try to remove a 
stuck sketch (cautiously or recklessly), but I 
can offer some clues as to when a sketch is more 
likely to be stuck to the cel in the first place.

(1) If the cel image is rather large (as in it 
takes up a good part of the cel), then the 
chances are higher that the sketch can be stuck 
to the cel:

More paint = more sticky surface area

(2) If the cel's image employs more darker or 
heavier colors (such as black, blue, etc.), then 
again the chances of a sticky surface area 
leading to a stuck sketch increases:

Darker colors = stickier back surface

(3) A really good clue for you:  If the cel 
exhibits tiny areas of paint loss (not line 
fading but actual holes in the paint), then it's 
more likely that someone tried to remove the 
sketch but bungled the job (obviously removing 
some paint in the process).

Paint loss <~> bad sketch removal job

(4) If the cel is particularly old, there may be 
a higher chance that it was paired up with a 
sketch for a long period of time.  The longer a 
cel is stuck to the sketch, the harder it is for 
the sketch to be removed.

All these factors should give you a better idea 
of when you're more likely to encounter a sketch 
issue and when you're less likely to.