On transmitters... the channels really don't have a neutral per se, they have a certain amount of travel that I'll just refer to as 0-100% ... I'm using percentages since different radios will have different resolutions ( the number of steps you have in that 0-100% range ). What is referred to as neutral ( when it applies ) is at 50% of the total throw on the sticks.
Set the trim tabs for the throttle channel to their center most position. If they are digital trims, you click them all the way up or down until they quit adjusting, then click them all the way in the other direction counting each click on the trim, halve the number of clicks to have the trim centered.
To check where neutral is at is pretty simple if you have a standard servo you can attach to the throttle channel. Using the longest servo arm you can on the servo, pull the throttle stick all the way back towards you and mark where the servo arm is located... now repeat the process moving the throttle stick away from you and mark it again. Then just move it to the half way point between those locations. I like to attach a long wire to the servo arm so I get a wider arc and can judge the center spot more accurately.