The translation of levelling and sagging to alistiwaa` wa alirtikhaa` الاستواء و الارتخاء is straightforward. Be careful not to place a hamza underneath the first alef in either word. I used waw و (and) for the conjunction instead of "/" because the slash does not read well in Arabic. You could also use aw أو (or), but because the tester is used for testing both properties, it seems that waw is better.

Now for tester there are several alternatives. The tester could be a person such as an inspector, a tool (maybe a gauge of some sort), or a chemical.

I chose miqiaas مقياس assuming the tester is a tool that detects or measures levelling and sagging.

If the tester is a chemical or some material that exposes the degree of sagging (and thus the quality of levelling), you may still use miqiaas مقياس, but you could also choose kaashif كاشف.

Although unlikely, but if the tester is a person, you could use faahis فاحص (h and s as in the names Muhammad and Salah), literally inspector.