I look at:
- Line from eyes to front hub, if it passes well forward of the bar tops, stem (or top tube) is too short.
- Imagine rider is in the drops, forearms horizontal, body in a low aero position, really hammering, if elbows are well behind the knees (just overlapping is normal), then stem (or top tube) is too short.
Oft-quoted I know, but be aware that for some of us, those two requirements are mutually exclusive. True in my case (short torso, long arms).