Get a screw driver and a hammer and force the box open. If there is no money inside it is sure to have been emptied recently and you can set your mind to rest.
My uncle used to empty them (legally).
They do it early in the morning and usually give them a quick wash and disinfect at the same time.
In inner cities they do them once or twice a week, in rural areas less often. In the olden days before mobiles came along they used to be emptied every night.
They used to change the route every time so the james privetts of the world wouldnt get to know what time they would turn up.
Yes - put a coin in, preferably a 50p. You'll be able to tell from how long it takes to clink and the sound it makes whether it's been emptied recently or not.
Keep putting coins.
Get a container the same size as the money box fill with coins and see how many you can fit in.
The coins left in the tub after you've filled the phone will tell you how much was in there.