Hi Norm. Try the stink test. Diesel and petrol have really, really different smells. Diesel is just a light oil- it's got a lot less volatile kaboom! vapors coming off it.
Match test a bit of the suspected diesel and if it won't catch fire, that's what it is. If it's really petrol, it will woomph into flame.
Other hand, if his "unleaded 91" was bought ten years ago, and has been stored for ten years in a leaky can.....
