How do you check the motor ground wire on a 94 Mercury Marquiis

what problems are you having?? you can use a set of jumper cables hook one end to the motor and one to the frame or body depending on which ground you suspect is bad

The battery ground is bolted to the engine and to the negative post on the battery.The connection must be clean (no rust, corrosion)and tightly secured.The computer grounds are also hooked to the engine.If you are having a no start or no starter activity you should charge the battery first.Then have a auto store do a load test to determine if it is still good or not.