If your hand is unable to reach inside an object and you want to measure the depth use?

A plumb bob (weight on the end of the string)on a string letting it reach the bottom then hold the string so it is tight/taunt then mark the string at the top of the  opening of the object. Then take it out and measure the string from the mark to the tip of the plumb bob(weight) this will give you a measurement. Or use a lazer(laser) measuring device.