Thanks to all who responded. The kero is coloured to start with so that method wont work(for me). I poured some into a saucer 8 days ago and nearly everything has evaporated. What is left is a very small amount of something!! I suspect it is Oil but I still cant be sure. I have just mixed up a fresh batch of Jet A1 with 5% turbine oil. I will do the saucer test with this and compare the results in due course. If I cannot be 100% sure I know I will end up tipping it into the domestic oil tank.Paul.