* 2-barrel: Will have a noticeably smaller diameter than a 4-barrel. It will have a single, smaller throttle plate.
* 4-barrel: Will be significantly larger and wider. You'll clearly see two throttle plates (often visible through a sight glass or inspection window).
If you can't see the carburetor directly, you might be able to tell by looking at the air cleaner assembly. A 4-barrel setup will generally have a larger air cleaner housing. However, this isn't foolproof as someone might have swapped components. The carburetor itself is the definitive indicator.
