If you can't find a button, put the lever in neutral and see if the handle will pull out away from the control about 1/2" if it does, move the lever forward, that should be throttle only.
There are so many different controls on the market, that without a picture of what you have, it's impossible to give you an exact answer of how to get to throttle only.
Older boats are even worse. You have no way of knowing it the control was replaced or not.