Axle fluid question

Yesterday I noticed the dreaded mark of dark oil around my diff. cover, and today I attempted to check the fluid level. The manual specs. 15-40mm below the fill plug for my truck. I can't get a finger in that deep around the pinion shaft...but I could get about 20mm in, didn't feel fluid on the end of it.

I don't know if there is a better way to check the fluid level.. I was thinking possibly a piece of string, but that gets complicated in that it would be hard to tell just how much string had reached into the diff. before it started going downward, how much bend it kept in itself, etc, etc.

Any tips? There was oil on the filler plug, so it obviously isn't empty, but I'd rather be safe than sorry.

You might try an allen wrench or a piece of wire bent at a 90 degree angle.

wire clothes hanger or a piece of metal or wood dowel rod. Mark the end where you stop inserting it so you have a place to measure from.