Actually it's really easy to tell, since there's just around a dozen basic patterns for small enemies -- you'll see them all in an hour of play, surely -- and the new patterns are obviously different, mostly far nastier. Numerical rank progression for the different difficulties is {1+4n, 12+8n, 20+12n, 36+16n} where n is floor(parsec # / 10). I think the threshold value for the new attacks is 250, but I don't feel like verifying it.
Don't forget that pretty much everything is fast bullets with a high firing rate. Sometimes the small enemies alone will trigger slowdown. You could just blindly tap-dodge through a lot of this, still, but then you'd stop scoring points.
A few examples:
