If one of your martials doesn't hit on a 14 on its first strike you can easily guess that the combat is going to be hard.
I disagree. Unless your GM is open about his dice rolls or the monster's AC and such, you won't have enough information after half a round. Luck will have more to do with the monster's efficiency than its level.
As a side note, the most deadly fights I've seen were nearly always starting the same way: A very tough monster rolling really low during the first round so noone uses resources just to ramp up the dice when PCs start to go low on life.
Similarly, hitting (or missing) on an 8 or 10 is also good for helping estimate enemy level. As a player, I find it is easy enough to guess enemy stats within the first round. Less so if the enemy goes last, but at that point you may as well have had a free round 0.
That said, I do wind up making it easier for my players to find this out when I gm since I make them roll for enemy attacks against them. There's a certain sick joy in watching their faces drop when they crit themselves or roll high for an enemy save.