I'd suggest having the event heal you all the time (assuming you're using Recover All rather than only wanting to restore HP), since it might not just be damage that the player has taken, but also loss of mana, states like Poison, etc.
However, if you want to do exactly what you were asking for, do a "Script..." event command and use the following:
$game_switches[33] = false
for actor in $game_party.members
if actor.hp < actor.mhp
$game_switches[33] = true
end
end
Then, do a Conditional Branch on Switch #33 (or whatever switch you feel like using as a "one shot" switch that events can freely turn ON or OFF for their own purposes) - IF SWITCH #33 IS ON should be the exact condition. Inside the first block, include the healing and appropriate text, and inside the second ("Else") block, have a Message that says "You don't have time to lie around!".