Okay here's what happen, a friend of mine bought a custom made cue, which has a standard pool size shaft, whereas the butt, he chose to have it smaller like 29.5mm - 30mm. And then when he got the cue, he played with it, he said it was awesome. And he played way better than when he used the house cue back then. then he hold the whole cue horizontally to check if its straight, and he said its straight. However when he'd do like most people normally do, which is roll the whole cue on the pool table, it was kinda like bumpy as in not straight, but when he roll the shaft itself and butt itself, its so straight. I tried playing with it, its fine, but the fact that its not straight when he'd roll it on the pool table bothers him so much, that its affecting his pool game. So yeah, what i think that caused this was because the butt was smaller and the shaft was standard size. So yeah, whats the best way to examine if a pool cue is straight or not.