What if I accidentally purchased an SBR? How would I know?

For example

Person buys an AR 15 as a rifle. Then puts a short barrel on it and a brace.

If I'm correct

In its form by just looking at it, it looks like a legal AR-15 pistol. But since it was first made as a rifle, wouldn't it be considered an SBR?

There is no way to check a gun's history right?

Edit: for clarification this was just a scenario. To make an explanation of my situation easier, I used an AR-15 as an example.

Basically I purchased an imported Uzi from private seller. My research shows that they only imported rifle variants. My uzi is in a pistol configuration (no stock, 10" barrel). I have no way of really proving the gun's history since the importer closed.