How to tell if someone is saboteur?
Obviously, if you are a digger and someone is continuously destroying your
tools, you might expect them to be a saboteur.
A less obvious
way is to detect who are diggers and use deduction. You know ahead
how many saboteurs there can be in your game so if you can eliminate
some players the chances for others to be saboteurs are getting
higher.  
Look at who is an obvious digger, and look at who is
destroying their tools, or blocking their paths. Obvious diggers are
easily targeted by saboteurs, so that is another way to spot a
saboteur. 
You could argue that in the end the best saboteurs are those that remain hidden
for a long time, but this could also be a weakness, if you are a
digger and you find it hard to detect whether a player is a digger or
a saboteur, maybe they are playing hard to read because they have
something to hide. Arguably diggers have no advantage having their identity hidden so a player who is hard to read is more likely to be a saboteur.


And if you still don't know, just call them a "saboteur" and see what the general reaction is. :-)