Butt weld two pieces together, then place it in a vice and knock from side to side, if the metal bends, great, if the weld cracks...
As said before...
Practice, practice, practice...
I bought a cheap Mig, cheap as in half price, from Halfords many years ago, It is great for non stressed items, that don't matter if they crack again, like an exhaust bracket, or a quick fix on the exhaust, BUT I would not weld structural items like sills or chassis or seat belt mounts, I don't get enough practice.:doh
I welded up the old lawn mower, the rot went 1/4 around the top, with a few steel brackets welded on, it seems strong enough. But I have since replaced it with a new one.
I also welded up the neck of a garden fork, seems ok, but might crack soon.
Years ago, I once tried to stick weld a mini sub frame....:doh
BUT by the time I found good metal, there was nothing left.:nenau
A new sub frame at the time was £25 but that was over thirty years ago.
But then, that cost was only part of it, by the time you replaced the brake pipes and all the bolts ....:augie