It's mostly by color and growth rate. Annua will be a much paler green, almost a chartreuse against any good bluegrass' blue-green (Abbey bluegrass is green with just a touch of blue, while Midnight is deep green with a lot of blue, almost edging into a phthalo mix). Both triv and annua will grow faster than bluegrasses.

Blade will vary by your type of bluegrass as well. It's wider than my Midnight, but a bit trimmer than my Moonlight. And, like any other grass...it has natural variants by your locale. Mine never has stolons because it's never allowed to go that far, and my lawn is too dense to allow them to develop.