You can't always tell by looking but you can make some good guesses.

This pumpkin is supposed to be 1400 pounds:

However, this one appears quite a big larger, and is only 828 lbs (official weight):

This was the biggest pumpkin at Carnation, Washington in 1998 and it was under 1000 lbs:

Looks to me like Rocky Pacilli grew an approximately 700 lb squash (it doesn't look orange enough to qualify as a pumpkin).  S'okay with me, it's larger than my best so far.  Enthusiasm is always good and as has been said, every pumpkin is over 1000 pounds until it's weighed...

@Man