I recently came across some great responses to this question on Reddit.
If you don't want to read the entire thread, I've pulled some of the best answers:
When a really drunk person starts to visibly drool they are about to spray vomit everywhere
A greenish tinge to clouds or a sudden drop in temperature or shift of wind can signal imminent hail or a tornado. If this happens, get inside or under something sturdy!
A sudden change in your boss or coworkers' attitude toward you. I noticed that my coworkers had suddenly become rather "cold" toward me for a couple of weeks, and my hunch was correct... they all knew I was getting fired and they were interviewing people to be my replacement at the time.
If the job you apply for/get offered asks you to pay for their inventory up front it's a scam.
If you're swimming and feel like you're going nowhere, you may be stuck in a rip current.
When corporate hires a consulting team to "increase efficencies in the processes."
When that happens, it is definitely time to polish up the resume.
My reasoning: if corporate leadership needs someone from the outside to tell them how to rid themselves of waste either A: they have no clue how their own business operates or B: they have no faith in their own management to get the job done or C: both.
All of which are good reasons to start finding another company to work for.
If someone tries to recruit you for a job (phone call, email, approaches you in public) and they skirt around the subject of what the job is, and try to get you to come in for an "interview" before telling you what the job actually is, it's either a scam or sales, generally 100% commission based.
If a friend has been depressed for a while, then all of a sudden is upbeat and happy with no change in circumstances, they could be planning suicide.
If someone is hot and stops sweating call 911, heat strokes are fatal and they're going to die if they don't get to a hospital very quickly. Also note: a heat stroke is caused by their temperature being too high not by dehydration.
If someone hads a terrible headache and pain when they look up or down that's possibly meningitis.
If a sober healthy person suddenly zones out and starts acting like a zombie or unresponsive and dazed while still being upright that might be a form of siezure.
If your mouth tastes metallic, skin tingles, there's buzzing/crackling from nearby conductive/metal objects, or all of your hair standing on end like in a movie/cartoon? You're probably about to be struck by lightning.
Is the local vegetation dead around a certain spot (incl mold/moss/etc)? There's probably a reason for that. Same goes for dead animals or animals avoiding something/somewhere.
Addendum thanks to /u/Doctah_whoopass: Also if there's a sudden increase in animal and especially insect activity, particularly concentrated in a small area, you might want to avoid it. Might not be dangerous outright but it's probably not going to be pleasant for you to get involved with whatever's attracting everything.
Nature is pretty loud, if it's quiet there's a reason for that.
The same goes for social gatherings and public locations. If a large crowd's attention is starting to converge on something and you're not at something like a concert or speech something is going on and it might not be good.
Following that if a crowd starts getting too dense/packed together (more than 4 people per square meter/ie when you can't turn around freely anymore) get the fuck out. At that point it takes one bottleneck or person tripping for something called called crowd crush or crowd collapse to happen, and when it does people die. Travel sideways and diagonally to get out of the crowd.
Are (faint) red streaks following veins coming from any sort of cut or wound on someone? Hospital time, blood poisoning is lethal. Catch it early.
Another important one /u/TokesMcSmokes reminded me of:
The "need to breathe" feeling is caused by too much carbon dioxide, not lack of oxygen. A lot of gasses and fumes will kill you dead with the only warning being dizziness or lightheadedness as you start to succumb. Seek fresh air immediately.
If you show up to a friend's party only to find a circle of chairs set up in the living room and a basket of plastic ware, you're about to be sold to for 2 and a half hours.
That slight, almost imperceptible shift in your SO's attention when you start to think they're cheating. When it's a creeping feeling more than anything you have real proof of.
A group of people no one really knows start showing up to a party when it's getting late.
If your partner all of the sudden starts accusing you of cheating without any solid reasons....could mean they are projecting their own cheating behavior on you.
QueueBehindMeWife suddenly getting new non specific hobbies and dressing better than normal to go to them. Spending more time on the phone, but walking away to make or take a call.
If you have to hide what you're doing from your friends and family, that's trouble.
If someone you don't know says they have a great investment idea for you, it's trouble.
If you have to ask if you should be doing something, it's trouble.
If a business cannot explain what they do/sell in one sentence it's probably a scam.
When a class room / lunch room gets very quiet when it normally isn't, a fight is about to begin.
When the tides start rapidly going back to sea, that's a good time to run. Away.