Actually, if you're pulling a lot of current, the voltage indicated at the battery may go well below 12V until the load is removed. I don't know how much a trolling motor pulls, so not sure if that's an issue here or not.
An ammeter is a point-in-time, right this instant measurement, so unless you have some sort of record of how much has been drawn, it isn't going to tell you much. Current drawn depends on throttle setting, supplied voltage, and any resistance in the system; not really sure that would give you any more info than if the motor is turning (which would be obvious anyway - if you get thrust, it's turning; if not, it isn't. If it smokes, you have a different problem.)