The high tech method of a broomstick (sans broom) with hacked markings at the 5L points all the way up to my pots max. Once you have marked to about 10L it becomes a simple measurement replication all the way up. Broomstick means you can stand up and measure without getting too close.
Im my HLT I used a spreadsheet to work out the volume using PI and all that.
Then just had a formula that gives me the depth in cm for each litre. I then printed and laminated it and it sits on the wall of my brewery, so I can wack a ruler in my pot and check how many litres I have against the table on the wall.
I have a sight glass in my kettle and have stuck on labels at each 2 litre mark.
I was probably going to just do bulkhead, 90 degree elbow, hosetail, silicone up to the top of the kettle and repeat at the top. But once you add all that together it becomes pretty expensive! However at least it is sanitary
I would be hesitant to put a sign gauge in a kettle, I think it may discolour fairly fast.
On my kettle I just put marks on the outsite as per here then eye ball it. If you are not that good at eyeballing it, then you could measure top of pot to liquid level, and transfer it to the outside to get an exact measurement.
My kettle is a 40 litre stock pot. When I first got it, I wasn't sure whether the 40 litre was the actual total capacity of the vessel. I used a marked 2 litre container to fill it with water, and counted the number of containers needed to completely fill the kettle.
Guess what? It took exactly 40 litres to fill it.
Next step was to measure the distance from the bottom of the kettle to the lip. It was made easy, because it measured exactly 40 cm. So, using a simple ruler means each centimetre equals 1 litre. If it had been different from 40 cm, it wouldn't be difficult to work out what measurement equalled 1 litre.
I used a length of flat copper, and bent it so it hangs on the inside wall of my kettle. I engraved it with my Dremel tool at 1 cm intervals, and also engraved numbers at 10, 20, and 30 litres.
Easy to see how much liquid is in my kettle at any one time. Simple.
After the sight-glass in my old urn-kettle got blocked, dirty and clogged with hop and break debris easily and often, it's not something I plan to have on the new kettle.
I'll just be using a marked length of pipe - as per suggestions above - so there is no worry about cleaning or clogging etc.
Like Truman I have a spreadsheet printed out for each of my vessels (hlt, 2 x tuns & 2 x boilers). i then use a stainless ruler to measure the depth & look it up on the chart. Its also useful for measuring mash in & sparge volumes - see what depth = the required volume, clamp the ruler onto the side of the hlt at that depth, then drain the hlt to the bottom of the ruler - exact volume in the tun. easy as.
If you have an android smartphone, download the BREWZOR calculator app. It has a calculator to do the same calks as a simple spreadsheet using pi but it includes temp correction.
Simplest and cheapest I've found so far is to buy a 90 degree bsp to comp fitting, stick a small length of copper in the comp fitting end, either weld a socket to the kettle or selfless with washer, oring and nut, then a length of the thinnest cheap silicone hose you can find clamped with a hose clamp to the small length of copper in the comp fitting.
Trouble with normal horsetails is that they are too long, this way you minimise the length lost to metal height.
Threaded Polycarbonate tube in a 90 degree elbow would be awesome but I don't have any
If you don't want to go with a sight-glass (which at the moment can be had cheaply from US), and don't want to make external pen markings there is another option. A cheap engraver from dirtybay will have you fixed in no time. Pencil them in the inside, and engrave away a quick line or X with X - 20L etc does the trick.