Is there any physical characteristics that will help me identify the difference between the two? Any help will be appreciated.

 

Weber never made a 51mm IDA. The 48 and 51 refer to the diameter of the hole down through the carb before the venturi. A 51 is just a bored out 48 if it is still using a weber casting. There are some clones out there that are pretty nice in that size range(50-52mm)
All it does is lets you use a bigger venturi than you could with a 48. There has to be some area with a smaller cross section to get the pressure drop to suck fuel. Most of the time a 51 is overkill unless you go with a PPort and rev the crap out of it.

 

You have to get a precision measuring device like a dial caliper, and actually measure the throttle plate bore to tell what it really is.

 

thanks for the information.