Troops with a general - preferably at least 6 troops in my experience though 3 may do for a while and 1 or 2 is better than nothing but that may only increase the risk you'll foil the mission, they'll find out who's there general-wise and then come back with an abduction squad (and with only a couple of troops may get through).
Fleet with an admiral (even better with commander+fighters and/or general+troops).
Self espionage backed up with sabotage/abduction resources (quite difficult against sabotages though as they're not there for long but you can get lucky). However, at very early game difficult to have these resources available (making obtaining them an early goal
Also depends on where they're coming from (assuming you know that or can figure that that out
Planet - could blockade it with fleet+admiral (at least, commander+general even better. Make sure they aren't there when you do it if you're fleet's a little on the weak side - perhaps wait for a successful sabotage against you, then nip in quick).
Ship in sector - try to chase the fleet out the sector (many like to park ships in neutral systems close by - good idea to patrol neutral systems and use cancelled espionage missions to detect incoming fleets)
Ship in orbit (blockading) - counter sabotage missions against the ship (meaning you want sabotage units on most planets)
Cant think of anything else at the moment - will post if I do