The first thing I do is go to the panel and check/tighten every screw in the panel, breakers and neutrals. Do not skip over double pole breakers as you could be dealing with a multi-wire circuit. Also check that all the breakers are plugged in good, I once disassembled a dryer for a failure to heat only to find out the breaker was not plugged in properly, the motor would run but it would not heat. After that, its time to map out the house and look for hidden things. Just because a receptacle is hooked up do not count it as good, backwired receptacles can be hooked to the wire and still have enough corrosion and such on the contacts as to kill them, check every one of them with a meter, or a Wiggy if you have one on the truck. The toner can be a useful tool, just be sure to get a good one, some will not pick up through sheetrock or plaster. This is a tough one to dive back in on, good luck.
