Look at the flat plate on the other side of the weld if you want to see if you got good penetration. You should be able to see discoloration where the metal was heated on the back side. Usually if you see that, you've got a pretty strong weld.

Also, if you are welding thick steel to thin steel you should point the mig gun more in the direction of the thicker steel. Instead of running the gun straight into the 'V' made by the joint, angle it a little more toward the thicker material. This will help direct the heat more into the thicker metal.

And when you are using a mig gun you want to 'push' the weld. Let's say you are welding from right to left. Angle the gun tip to the left, and zig-zag the pattern from one metal to another. Above all, be patient and go a little slower than you think you need to.

If you want to be a better welder, practice a lot on scrap metal, and try different things until you get a nice weld. Good welding takes practice. You might also want to take a welding class at a tech school.