uname -m
NOTE: It shows if your system is running 32-bit (i686 or i386) or 64-bit(x86_64).
Example 1:
prashad@ubuntu:~/Desktop$ uname -m
x86_64 <<<<<<<<<<<<<<this is 64-bit
Example 2:(you can check with 'lscpu' command also)
durga@ubuntu:~/Desktop$$ lscpu
Architecture: i686 <<<<<<<<<this is 32-bit
CPU op-mode(s): 32-bit