The technique of drawing a stocking or Q-Tip through the guide or tip top to check for grooving, which is used on plug and spinning rods, is not useful for fly rods. Fly lines don't usually wear grooves. Instead, they wear the finish off in a flat manner. The fragile stocking or Q-Tip is not likely to pick up on this worn area.
The best method is to use a magnifying glass and examine each guide. Rarely will the ceramic stripping guides be worn, but the snake guides and tip top will wear. What causes most of the wear in fly-rod guides is a dirty line that rubs abrasive material against the guides. Keeping your fly line clean will help postpone the need for replacement guides.