Older type machines can be broken by drilling a tiny hole in the right place and you can use a thing bit of wire to flick the coin tubes and coins will drop out. However, these types of machines are few and far between and all new machines have hoppers instead.
The refill key only works on a few machines, older style Barcrest ones and the only thing it does is tell you how much is in the hopper, it doesn't affect any winning percentages. If the machine is full, there's more of a chance it will pay out, but it's entirely possible it was filled by hand earlier that morning. That means that whilst it's showing as full, the amount gone in is less so the payout percentage is obviously lower. If your machines gets played a lot, then it will payout more, the payout limit has to be set at 70% over the life of the machine. If it's a new machine, the payout may be say high to entice new players
Regular players of machines know which machines are worth playing and where they are. Machines in pubs are worth playing on a Saturday or Sunday morning as drunkards have usually been playing the night before and lost a load of money so there's a chance they'll pay out. It's a tactic I use and one reason I never play them when I've had a drink.
To the best of my knowledge and I worked in the arcade business for several years (albeit a few years ago) there is no way of fiddling with the newer style machines, short of hacking them open and hooking a laptop up to them.
At the end of the day the machine will always win, they'll always take more money than they pay out. Ask the manager of the pub if he's having to refill it a lot more than usual and then keep an eye on it if you're concerned