Whether or not an engine is on condition is way down the list of thing syou should be paying attention to when renting an aircraft. As others have mentioned the airworthiness of the entire airplane is far more important.
That means before walking out to it, knowing its inspection status for both scheduled calendar and hour based inspections as well as out of phase inspections like AD,s as well as deferred defects. Then having a good look at it during the walk around inspection to make sure that the aircraft has no obvious problems.