I run tubeless on my mountain bikes - it rides better, it flats less, it's just awesome. (I still don't find road tubeless to be ready for prime time unfortunately) One of the challenges of running tubeless is making sure the sealant hasn't dried out and there's enough left. Most of the time it involves pulling one tire bead off the rim and looking inside, but that can be messy and usually is best done with the wheel off the bike. In the past, I've used a spare bike spoke and dipped the threaded end into the tire after removing the valve, and that actually works pretty well, but not everyone has (a couple hundred) spare spokes on hand to do this. I recently saw that Orange Seal has a dipstick which can serve the same purpose... but man, that sure looks like something that can be printed.

So, here's my take on a tubeless sealant dipstick. I have two versions with a different length, the 95mm version should work with most non-fat MTB tires and all road tires (unless you've got really deep section rims and long valves) and there is also the 125mm long version (which just fits on the 120mm print bed of a Monoprice Select Mini) and a 200mm version. If you need something outside those sizes, I've also included the Fusion 360 file so you can update the length in Sketch 2 to whatever size makes sense for you.

I designed it with the small ridges at the end to catch a little sealant and hopefully show the level better, I found that the threads on a spoke accomplished the same, and I can't tell if the Orange Seal dipstick does the same. I'd also suggest printing in a color that contrasts with the sealant you're using. I printed in black PLA.

Update: Sep 1, 2018:

• Realized that these things needed a provision for hanging on a pegboard hook - so I added it.
• I work at a bike shop and we got the Orange Seal kits in stock that have the dipsticks, and they do have a ridge for catching the sealant, similar to this design.