If there's no blood in the syringe when I aspirate, my gear is going in whether or not I feel the needle has passed through a vein.
Most of the time it's just scar tissue that makes me wince. Test, E.Q or preemo never made me cough. I've never tried tre.n...and probably won't.
If you hit a vein and didn't pass through it blood will rush into the syringe, so it's pretty obvious when not to inject. Don't be nervous if blood comes out of the injection point after pulling out the needle. I've actually had blood shoot out of my quad before, it was very entertaining...shit was a fountain shot that flew about 6 inches in the air lol.