I repair donated recycled lights as a hobby. Even got written up about it in the local area newspaper.

Biggest problem with the LED light strings that I pick up is the actual wire coming out of the LED rusts and doesn't make good contact with the socket.. Sometimes, when you pull out the LED and the plastic socket, the wire breaks off inside the plastic socket. Sometimes the broken wire lead falls out, but sometimes the rust holds it in the socket and everything looks ok. Patience and a good game on the tv works out best for me.

I also have a variable voltage source, a variac, that I set at half voltage or lower, and then use a wire jumper to bypass half the string of bulbs. If the sting lights up, the bad bulb or connection is in the bypassed half.. If the string stays out, the bad bulb is in the unbypassed lights. I reverse the bypass and see if the string lights. If it does light, then I have verified the general half of the string where the problem is. I then bypass half of the defective half to more closely locate the problem. I keep on doping this by halves until I get to the bad actor. This works fine as long as there is only one or two bad actors. If a lot of bad bulbs , good luck!!!