Well, if it is a 400, that is a factory displacement, and would be indicated by the numbers on the block. You can pull the numbers off the back of the engine (at the very back of the block, behind the heads, by the distributor, I can't remember which side) and run them at
www.mortec.com.
Displacement on smallblocks is hard to determine without looking up the numbers. As magnethead said, sometimes (like with a 383) you can't even go by the numbers because the engine is made from one kind of block and another kind of crankshaft.
Because of this, it is easy for sellers to lie about what is in their vehicles when they sell them to try to raise the value. Some folks aren't telling lies, they are just repeating what they were told when they bought it.
In other words, don't be surprised to find out you have something besides a 400. If you don't, hopefully it will be a 350 and not a 305 or an old 307 or something. The best way to look at it is that you have a good, strong running smallblock regardless of what it is.
Slonaker