Sure, PCs going down is a definitive sign that the encounter is pretty hard for them, but do not make the mistake to assume the converse. Sometimes a won encounter appears to have been easy when you just look at the result (nobody dropped, not too many HP lost etc), but that doesn't account for the possibility that maybe you only rocked the encounter because that one die fell in your favour, and if that roll had come up differently, it would have set events in motion that could have even led to a TPK.
Like, reminds me of that one skirmish we once had in PF, level 7 or so, against a bunch of Chuuls and Rorkouns (slime-spitting worms). There were just 3 of us, which generally means there is little margin for error. So on this occasion, one of us is busy getting an overly attached Chuul out of his hair. Meanwhile our Bard makes a little mistake with her movement and exposes herself to a Chuul. The monster does a lot of damage, but fails to Grapple the Bard due to a Nat1. Then the Bard manages to Tumble out of range, barely rolling high enough to avoid an AoO. This happened right in the first round before we could get any buffs up. After that, we get our act together and shred the saucy seafood to Nigiri-sized chunks.
If the Bard had gotten grappled, she would have been paralyzed. If she hadn't managed to tumble, the AoO would have dropped her. Without her buffs, the other PCs would have gotten zerged, grappled, paralyzed, dragged under water, final curtain.
So in short, just looking at the outcome you might think "Easy, just some HP damage", but actually it was on a razor's edge due to one little mistake (made by our least experienced player), and our bacon was saved just by two lucky rolls in a row. Then again, it was a CR11-12 encounter while our APL was just around 5.