>>395127251) If you are able to install some apps on the phone before buying it, install AIDA64 and check in the cpu menu if there's any mention of "32bit" or "32bit mode" for the "instruction set" row, if one of these two mentions is present, this phone is currently running a 32bit OS
2) If you can't test on device, you can also Google search something like "[phone name] 32 or 64bit OS?" but it's way less precise and probably won't work for relatively recent or semi-obscure phones
3) To be 101% sure, buy a phone with MORE THAN 4gb of ram (not simply 4gb, MORE THAN is mandatory), with Android 8.0 or better on it. This way you will both futureproof Google's more aggressive 64bit policy coming in 2021, and enjoy the benefits of project Treble (os updates coming faster and over a longer period of time)
A 32bit Android OS can't handle more than 4gb of RAM, so all phones with more than 4gb of ram have to use a 64bit os
