Mysterious MPG drop

Am experiencing significant MPG loss (50MPG per tankful) over past couple months. Have replaced air filter, spark plugs, spark plug wires/coils and added injector cleaner. Have read manual & online tips on checking MAP, O2 sensors & mass air flow sensor but they are unclear: I can't tell which wire is which on the connecting plugs to run a multimeter/voltmeter test. Am going to remove & clean the mass air but that's my last hope other than just start replacing things and I'm on a pretty fixed (low) income so I'm trying to do it myself - any suggestions would be appreciated