How can I prevent insects from entering my house? How can I find their entry point?
Get an IR camera.  You should be able to find cracks/leaks/whatever real easy.  Unfortunately IR cameras aren't cheap, so perhaps you can call a HERS guy to come over with one and pay him $100 for his trouble.

If that fails, I suppose you could fill your house up with a smoke machine and attach a blower door (you'd need the help of a HERS guy again)... or just use a smoke pencil.