Its (usually) *NOT* the PE wiring resistance that's the problem. Soil conditions and ground electrode depth and surface area are the usual limiting factors. For a DIY methodology for ground resistance testing without specialized test equipment see '2. - Using a transformer, ammeter and voltmeter' here:
https://www.tlc-direct.co.uk/Book/8.6.1.htmThe temporary electrodes and hookup wire don't have to be durable or of the quality of the permanent ground electrode so you can use cheap steel pipe or steel stakes, simply clean off any grease paint or rust immediately before use, and connect the voltmeter U directly to or as close as possible to X, the ground stake under test.
The IEEE is of the opinion that TT system loop resistance should be under 200 ohms, so worst case, for testing, you are looking at having to drive a substantial current through twice that. OTOH for safety you don't want to go over 50V and personally I wouldn't use over 30V. The transformer *should* to be able to deliver 1A. Put one or two well chosen car bulbs, or a power resistor, in series to limit the current to less than the transformer's secondary current rating, and you can get away without having a power rheostat.
The US NEC requires an additional ground rod if the ground resistance is over 25 ohms, (See
https://www.mikeholt.com/PopGraphic.php?id=4134 ) so that's a good target figure to have in mind.
N.B. re-test it at the end of your summer or dry season!