Well after several annoying attempts at tapping the side of a keg (much like kicking a tyre), messing around with scales (meaning disconnecting) and generally just faffing about, I decided there has to be a really simple way to see what's left in your Corny! Well good news there is, and it involves eating chocolate and drinking beer! 1. Buy a Kinder Surprise Chocolate Egg 2. Pop a small magnet in it (one that looks like the old fat watch batteries), you don't need to stick it down as I have. 3. Drop the food safe (I'm assuming Kinder use food safe plastic here!) egg with magnet into your beer filled, pre carbed keg. Close it as normal. 4. Grab a larger magnet and attract the Egg inside to the edge of the keg. 5. Drop a steel ball-baring onto the magnet and slide the larger magnet way, leaving the ball-baring holding the floating Egg in place. 6. Each chocolate, and test ball-baring movement by drinking beer. Hope this help, and let me know if you find better bits to do this method with. I am currently using a BB gun baring which could do with been a bit bigger but you get the idea!