I guess I've used used a few different types of optics.
As noted, the really cheap optics won't hold zero, and have parallax issues. I had one cheap micro-dot that would move several feet off the target depending on where the eye was in relation to the optic.
To check for parallax, put the optic on a table and align it on a target. Now look through it without touching it. As you move your head around, the dot will appear to move within the lens, but it should not move off the target downrange. With a good optic, the dot will stay on target, no matter where in the reticle it appears. With a bad optic, the dot will wander all over around the target as you move your head.