http://billiards.colostate.edu/bd_articles/BD_StrokeChecklist.pdf
Pay particular attention to the Centerline Stroke Drill. If you can do it 10 to 20 times in a row successfully, (bringing the impacted ball precisely back to your cue tip, your stroke is plenty straight enough no matter what's taking place before your tip impacts the CB (or striped ball in this case, used as a more revelatory, test-purposes surrogate for the CB).
Don't underestimate this CSD as a virtual descriptor of stroke straightness and centerline hit. Steve Davis would unceasingly perform this drill for two hours on end before every snooker tournament. A ritual he maintained for years, saying that of all cue sports disciplines, snooker for obvious reasons, had the highest demand for the straightest of strokes of which the competitor was capable, and this drill -- however arduous, was the single best means of achieving and *confirming* (to specifically address your question) that championship-level of straightness.
Arnaldo