Can anyone tell me how to tell?

between the 2 that I have seen,,, the original had crappy welds

That is what I was going to say! Whoever did the welding on the factory K frames didn't know how to weld. They must have hauled in drunk guys from under the bridges and told them to weld K frames.

Any other markings on a OEM one beside bad welding job?

If you really want to spot a reproduction examine the welded on motor mount areas, look at the sides of the fromed steel parts that would be added to form the motor mount bosses. These pieces are generally reproduced using modern day manufacturing techniques such as plasma, laser, or water jet cutting which makes the edges look different that originals which were stamped using dies. The edges of the die stamped pieces generally have a pinch line along the center wher they sheared in the stamping process, the modern edges look more serrated like the edges of a dime or a quarter. This same method can be used to spot many modern reproduction steel parts such as intake manifold brackets and motor mounts.