3 common ways include:
1. calibrated dipstick or spoon (easiest)
Before your next brew session, take a 1 gallon pitcher or some other known volume vessel. One gallon at a time, pour the water into your kettle, let it settle, then put the dipstick or spoon in. Mark the top of the wet line with a sharpie or craft knife. Repeat for each gallon. Measuring volume is now just a case of putting your dipstick in the boiling wort and reading off your volume.
2. calibrated sightglass (harder)
Drill a small hole at the bottom of your kettle and using a bulkhead fitting and street elbow create a 90° turn pointing upwards. Install a polycarbonate or glass tube into a compression fitting. Now when you pour liquid into your kettle the level will be revealed by a column of water in the polycarbonate tube. There are kits you can buy for this but beware using these on kettles as too much heat applied may melt the polycarbonate or boil the water in the tube. Glass tubes usually need some kind of protective shroud so you don't smash them. Sightglasses are perfect for vessels that are high up like a hot liquor tank where you typically can't look down into them. Calibrate as in option 1 - dipstick.
3. calibrated float-tube (novel)
I saw this idea recently and remain intrigued. Braze/solder a stainless or copper tube into your vessel with a hole at the bottom to allow the liquid into it. in the top of the tube insert some kind of buoyant rod, like a balsa wood dowel. As the liquid in the vessel rises it pushes the dowel up and this can then be calibrated in the same way but your markings are now in reverse, i.e. the 5 gallon mark is at the bottom of the dowel, not the top.