Nothing says amateur like an accidental crooked shot.

So, how do we keep this from happening. It is pretty simple really. Get a tripod with a leveling bubble (or two) and use it every time you set your tripod up in a new location. See, that was not so hard. For those that have a tripod without a leveling bubble... yeesh, get a new tripod. I’m kidding really, this tip is actually for you.
My first three tripods didn’t have leveling bubbles. I really just could not afford it. But that was a long time ago and leveling bubbles are not only on high end expensive tripods anymore. Budget tripods can have them too. Just make sure to look for this feature when shopping for a tripod no matter your price range.
Of course some of you may already have that tripod without the bubble. What if your not even using a tripod. Hand held shots are also very susceptible to off-level shots too.

OK, so here’s the secret to leveling your shots without the bubble. Look for strait vertical lines. The side of a building, windows, a telephone or power pole. Anything that you are sure gives you a strait vertical line. Don’t use trees, they’re not always perfectly vertical. Of course power poles aren't always straight either. Just find something you know is straight. Then just line up the edge of your viewfinder frame (another vertical line) with this edge. You can also use a horizontal line with the bottom or top edge of your viewfinder. Keep a lookout for these lines of reference all the time and it will become natural with practice. Even when I worked as a news photog. If my camera was on my shoulder I always kept my eye on the subject and leveling lines.
If you’re using a wide angle lens then all bets are off. Vertical lines get distorted. At this point you just have do make your best guess but if you are keeping in mind that you need a level shot you should be able to work it out.
Good luck and keep shooting.