Most routers I've worked with will list the NVRAM amount in the output of show version. It's usually after the interface types.
from a 2620:
Processor board ID JAD06170E1H (3632925778)
M860 processor: part number 0, mask 49
Bridging software.
X.25 software, Version 3.0.0.
1 FastEthernet/IEEE 802.3 interface(s)
2 Serial network interface(s)
32K bytes of non-volatile configuration memory.
16384K bytes of processor board System flash (Read/Write)