Its very hard to tell, and almost impossible with white diesel. At least with red diesel, the colour will change from a dark red, to pink, depending on how dilute it is.
Another worry with places selling cheap diesel, is some people use acid to remove the dye from red diesel, and then sell it off as white/clear. This will also prove very harmful to your fuel system.
Im sure there are local government, or perhaps fuel retailers places about where you could possibly get a sample of the fuel tested for quality, at a reasonable cost.
It also make little sense at the minute anyway, diluting red diesel, as chances are its almost as cheap, if not in some cases cheaper than heating oil.
The only thing with red, is that depending on where you buy it from, regular fuel filter changes are a good idea, as perhaps some of the storage tanks arent as clean, but it depends on where you buy it from.