32-bit and 64-bit Android: how to tell before buying a phone
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