I understand your frustration and some times being shot in the back or picking the wrong corner of a room to check first cannot be avoided. Bad spawns happen. But, there are still many ways to limit the occurrences of things like you described happening.
There is actually a lot of little things that go on in a game that can help with the problem. For example, you can lead into the room with a stun/flash. If you get a hitmarker, you know someone is there. Check the corner you can visibly see without fully entering the room, then make your way around to the other entrance to the room to check another corner. It may seem like a lot, but it gets the job done often. Of course, that is situational and won't be 100%.
Pay attention to your team mates locations at all times, as this will dictate where the enemy will spawn. Yes, some times the game will be evil and spawn them behind you regardless of team mate location, but nothing one can do about that. This also will depend on game type being played, as in games such as Domination, flag ownership will play a huge part in the spawns.
Pay attention to the death skulls that pop up when a team mate is killed, it can be very helpful in giving away enemy location.
Try not to sprint when it is not necessary, being caught in sprint by an enemy is a sure way to lose a gun fight.
When capping a flag in dom, be aware of the popular locations in which enemies like to come from to defend a flag. Don't hesitate to turn around quickly to throw a stun/flash to check the area for a incoming enemy. You can also purposely hit the flag so that it tells the enemy team it is being taken, and then proceed to take cover and wait a second to see if an enemy is near to defend it.
Deciding to pursue an enemy around a corner is different for each situation. If you put shots on him already, and have him hurt, it may not be a bad idea to round the corner after him. But be prepared for him laying prone, perhaps even pre-fire the corner in anticipation of him being there. Also, gauge your competition in the match. If you are playing with good players, you can expect that type of trickery. If you find yourself playing against bad players, chances are he's not laying and waiting, he's still running and will be an easy target.
If you are in a game where the enemy team controls all 3 flags (in which case your team is spawning everywhere), be aware of this fact. When you spawn, check your back to make sure you haven't been spawned directly in front of an enemy. Of course, this won't always work and you'll still get killed, but the idea is to cut down on how much it occurs.
So much little information is given to you during a match to help you with the decisions. For example, you can tell by the way a guy is running if he has a shotgun. This should change how you approach the guy as you know you have a range advantage assuming you aren't running a shotgun. Or when in a gun fight, if you take note of the type of gun the guy is using, you can know how many bullets he has to work with. You can then estimate how much he has fired at you to use that to your advantage, knowing he'll need to reload.
Combining all of the above with situational awareness can go a long way to preventing a lot of frustrating moments. Take note of your competition, the game type, the spawns, the map, try to use it all to your advantage.
