32-bit and 64-bit Android: how to tell before buying a phone I have been looking into 32-bit or 64-bit Android versions for phones. It seems to me that the information on the number of bits of the installed Android version is always missing from phones' specs, unlike what happens for the CPU. Googling around, the ways I have found to understand whether a phone is running a 32-bit or a 64-bit OS all include actually doing something with the phone, like looking up the kernel version or downloading some app, like AIDA64. 

How do I know if a phone I want to buy will come with a 32-bit or a 64-bit Android before I buy it? It would be interesting to find a layman-friendly criterion or resource that allows me to know this in advance, especially to be able to look into the cheapest phones running a 64-bit Android.
One way to know that the OS is 64-bit is to check the RAM. 32-bit OSes have (see/utilise) a maximum of 4GB of RAM while 64-bit devices see more.
Devices with more than 4GB of RAM will be using 64-bit Android.

Limits are as followed: 

32-bit:  2^32 = 2^2 * 2^30 = 4GB. A 32-bit OS will go 1 bits up to 4GB.

64-bit:  2^64 = 2^4 * 2^60 = 16EB. A 64-bit OS will go 1 bits up to 16EB.

Up to 4GB, the RAM won't help differentiating whether it is 32 or 64-bit. More than 4GB, then it is more than likely to be 64-bit.

Large Physical Address Extension(LPAE) was implemented to allow physical address up to 40-bit. But as @Irfan as inferred, it is not known (at least publicly) if there are devices in the market which have taken advantage of it.

Conversion table
-----------------
2^10 = 1 KB
2^20 = 1 MB
2^30 = 1 GB
2^40 = 1 TB
2^50 = 1 PB
2^60 = 1 EB