Is there some way to tell if you fuel has lost ether?

Can you smell it when you enter the area of the room where you store your diesel fuel? If not, and unless it's completely gassed off it's probably ok. Some people mark the fuel level in the bottle with a bit of masking or electrical tape. There is some variation to be expected in the height of the fuel between seasons and changing air temperature. So the latter may not be a useful measure.

Best bet is to mix what you need on the day :-)

You can decant 100ml of your fuel into a measuring beaker and let the ether in that evaporate off.
What you will have remaining is kero and oil separated and able to be measured. You can then calculate how much ether was in the mix you put in the beaker, and this will tell you if any had left your fuel mix.

Funny story, as a retired anesthesia doc I assumed I would have no trouble getting ether to mix my own diesel. Wrong!!!! MD/DEA license or not the company would not sell me any ether. The comment was (correctly) that 1)ether has not been used in the US as anesthesia for many years 2) I do not possess a chemist's license which would have gotten me ether and a whole host of other things I imagine 3) What could a doctor possibly want with ether? My explanation of home made model diesel fuel did not convince anyone. Wonder if I am on yet another list in D.C. because of this query.

One simple source for ether is John Deer Starter Fluid. This is ~80% ether. I like just mixing the JD with mineral oil in a small jar marked with the correct volume of each (25% oil). This fuel uses a bit more then kero based fuel (~ 15% more), but it is simple and always fresh. Some other Starter Fluids will work mixed this way also do some test mixes and try it out on a test bench to get the correct prop and compression settings.

Best, DennisT