I'm assuming you mean cut-to-length plate from coils that has been flattened.
How you know if it's okay is past experience using that product.
I've known of fabricators that would use it for roofs and bottoms and not for shells, and others that would use it for shells but not for roofs or bottoms.
The issues are flatness/waviness in the product as received, and possible increased distortion when welding due to residual stresses.