Because it’s hard to quantify this (priming a diesel is so tricky on an unknown engine, and diesels flood kinda easy), but I would say a hard starting engine that normally is easy to start would be a reasonable indicator. If you’re questioning the fuel, grab an engine you know well and starts easily and use it for your test mule. I know from previous experience that fuel with too little ether requires a higher compression setting (than normal) to start and run, and it tends to be a bit more “rattly”. I use the previous “run” compression setting as a good indicator. If I need to raise compression more than, say, a 1/2 turn or more, it might need a spritz of ether.