I mean the dispersion of the grains in alloy system. How do I locate the grain boundaries and determine the actual size of the grains?

There are standards:

• ISO 643:2012 Steels -- Micrographic determination of the apparent grain size
• ASTM standard E 112-96 standard test methods for determining average grain size

Also you can find additional information here

https://www.olympus-ims.com/en/applications/grain-size-analysis/

and here

https://www.researchgate.net/post/How_do_you_measure_the_grain_size_in_metals

Please have a look for a well written paper of Dr. Bernard M. Collett which briefly explains about SEM.

Attached is the file which might be useful for you