I always analyze any encounter I design against the characters I will run them against. To me this is just good GMing. What I do is figure out how long it will take the players to beat the NPC. Then I look at how long it would take the NPC to beat the players. I run both sets of number with and without buffs. This gives me an idea of how tough the fight will actually be. As long as the players have a reasonable chance to beat the encounter it is usually good. This does not guarantee the party will win, because there are a lot of things that cannot be planed. But it help to spot when I have poorly designed encounter.
What is an easy encounter for one party may be a TPK for another even if they are the same level.