Opening the door just a crack and readying an action to slam it shut should there be hostiles makes scouting doors a lot easier.
Keep in mind that you can't ready actions outside of combat. The one time that this happened in particular, it went like this:
Rogue: I crack open the door slightly, and I peek inside.
DM: You don't see anything. (all of the enemies had heard the combat in the previous room and had taken up positions in the room that couldn't be seen from the doorway)
Rogue: Alright, I open the door and sneak inside.
DM: Ok, you get about 10 feet into the room and then a bunch of gnolls jump out of their hiding spots. Roll for initiative.
We once had a situation where the Rogue could have spotted them from the door, it went like this:
Rogue: I crack the door open slightly.
DM: You see a bunch of Gnolls in the room. Roll for initiative as they all all watching the door and see it crack open.
Rogue: But I made a 29 Stealth check!
DM: Great, you made a 29 Stealth check, but the door doesn't get to use your Stealth check. They see it move. It's about DC 0 when they are staring at it.
Rogue: Well, then, I close the door and quickly run away.
DM: No problem. You can do that. If you beat them in initiative.
It was then followed by a Gnoll opening the door before the Rogue acted and the rest peppering him with arrows. And the Rogue dying.