You would need to ask for a detailed SoH check to get an even remotely accurate reading.
The variance on a GoM is massive and is dependent on so many external variables that it is useless to determine battery SoH (not slightly, or ball park, or indicative, just totally useless). You could be out by a magnitude far greater than what would be considered acceptable variance. Think double digit percentage points out.
To put it in to perspective. Most EVs might have typical battery degradation of 3 - 5% in 5 years. On my EVs 3 - 5% is about 10 miles of range lost. I can get the same loss by just turning on the AC. Any actual degradation on a battery would be lost in the noise of what a GoM gives as an estimated range, based on previous driving styles and conditions.
You could drive to 0% until the EV literally dies, then charge to 100% and calculate the energy added. Then drive to 0 again and calculate the range, energy used and the efficiency to determine net battery capacity after charging efficiency loss. But even that depends on the OAT and other factors and is of course very time consuming. Or to put it simply, not very accurate and could still be out by a few percent.
In short… a detailed battery SoH check is quicker and the most accurate method when looking at used EVs. Always ask for a SoH check and ask for a hard copy.