There's a papyrus function Game.GetPlayer().GetLevel() for scripts or simply a GetLevel condition function you can run on the player. You can look at some of the daedric quests for examples how its used (DA##QuestName or something) as many of those require the player to be a certain level to trigger.
If you want to start a quest as soon as the player reaches a certain level you can use the Story Manager Event Node "Increase Level" and set the condition to the wanted level.