What is the best way to check for leading?
Visually inspect the bore. Leading in certain areas of the barrel are usually tell-tale signs of certain problems (ie leading at the end of the bore usually means a boolit may be pushed too hard or inadequate/not enough lube). I have found that if I don't push some boolits hard enough/or they are sized just a little too small, they will lead at the beginning of the barrel...maybe just a quarter inch or so. If a boolit is grossly undersized, you may have lead strips in your bore because the boolit could not full engage the rifling and the rifling strips the lead right from the boolit. Another sign of this is leading at the leading edge of the rifling.
The key to success with lead boolits comes doen to SIZE. Most guns prefer lead boolit to be .001" (sometimes even .002") OVER GROOVE DIAMETER. This means you need to slug your bore to figure out where you need to be sizing your boolits. Good luck!