Learn to tell the difference (I know the film is called The Two Towers, but I know spires when I see them)
When you come across a thin building taller than 6 stories high, do a quick check to see if it is a tower. If it is then you have no problems, provided you are a law abiding citizen. However, if it could be a spire then you are in trouble; it is undoubtedly inhabited by an evil wizard or the matriarch of an alien insectoid race. It's probably too late to run.
This a tip for the the mechanically minded. Find an excuse to look under the hood of the nearest powered vehicle. If you see a motor or an engine then you are sitting pretty, this applies to anything from the smallest Morris Minor to the space shuttle. Woe betide you if you spy any kind of drive. Although the vehicle can probably fly or travel trough time, drives are notoriously unreliable especially during the inevitable alien attack.
This princess doesn't know how to ride in a boat, and also smells like onions 3
If you ever find yourself in a forest, jungle, or otherwise surrounded by vegetation of significant size, take a minute to look around. If the foliage is green, or even lush you are probably OK, but if it is verdant then the undergrowth conceals lurking enemies, or is itself dangerous. One exception - if any of the leaves appear to be sun-dappled then you will probably soon come across a rather dopey princess riding a unicorn or something. In either case, my advice is to run like hell.
An Experiment: Find a space that does not get a lot of fresh air, like a seldom-opened wardrobe or garden shed, and take a deep breath. If the air is musty or stale you are OK. Dank air is cause for concern, and if the air is close then watch out!5
Finally, if you are not squeamish, you night like to try a quick autopsy on any recently killed animals or humanoids. Pay particular attention to the fluids that escape from the body. In the real world you will see mainly blood (red) and possibly bile (green or yellow) - this is a good sign.
In contrast, fantasy creatures are filled to gills with sticky black ichor, often under great pressure so the merest prick with a blade results in a great shower of the stuff. If you see any then watch out, since the corpse will most likely rise again soon.