I’ve had this body tape measure and fat caliper set for years. They’re easy to use, last a long time (for me) and are pretty cheap.
It’s comes w/a set of instructions and a table/chart to show you, based on the caliper measurements, your body fat percentage.
I’ve used the tape measure for my arms and other measurements to track them in general, but also for change during programs I’ve done (e.g., P90X).
Also, there are some body fat calculators online that use measurements like the neck to calculate.