Easiest way to check for spiralling is at night with a good shooting torch, against a dark background,
no need for a target just shoot over a field towards a bank/hedge 50yds away, the light should reflect back from the pellet skirt recess allowing you to watch it's flight path,
it's easy with .22/.25 but with .177/.20 you might need to paint the skirt recess with tipex correction fluid to be able to see it.
I think it's more likely to be scope/rifle canted, a good test is to draw a + on the target with a spirit level so the lines are exactly horizontal/vertical with the target mounted & use those lines aligned with the scope reticle cross.