We all need to be prepared for certain things in life: debt, pain, stress, the last piece of cheesecake in the cafeteria, and alien invasion. We will all have to deal with these things at some point. However, I am afraid that people are woefully unprepared for a secret alien invasion. I plan to correct that now.
First, allow me to clarify the kind of alien invasion I am talking about. I am not speaking of the kind of obvious attack you so often see in films. The military and everything are going to do their best to combat that; there is little you, as a private citizen, can offer as far as aid. Let them do their thing. They have the big guns. However, there may very well come a day when the invasion is here, under our nose, and the military and the government don't even realize it. This day might be tomorrow, thirty years from now, or it may even have already happened. This has been the subject of several notable science fiction stories. The point is, in the event of an alien invasion through subterfuge, you must be on your guard and ready to spot the aliens and thus bring them to the attention of the world at large. Only then will we have a fighting chance.
So, for your convenience -- and I hope you treat this seriously -- I have assembled a small list of tips and tricks for you to begin to apply in your daily life. Remember: they are out there, they might be among us, and it might be you that needs to stop them.
They may be able to take human form.
In fact, they probably will be able to. You cannot expect to see any physical signs of their true nature. They will undoubtedly look completely different from us in their true forms. Therefore, they will have some way of disguising their true nature, either through a cloaking device, shapeshifting abilities, or any number of technological methods. You will not know them by sight. They could be anyone, even your best friend. Paranoia is very healthy in this situation.
They will be unfamiliar with human culture and mannerisms.
As a result of being from a different planet, they will not be able to pass completely as human. As a species, we have developed many things, such as body language, that are second nature to us, but will be very difficult for aliens to pick up on and imitate. Expect aliens to be socially awkward or to react oddly to certain situations. They will often have body language that feels forced or oddly controlled. You will pick up on this subconsciously. Listen to your instincts! A big tip off will be laughter. Laughter is a very human trait, and an alien who tries to imitate it is very likely to not get it quite right. Pay attention to people's laughs. Are they very forced? Do they only have one or two laughs that they ever use, as though they have practiced them? Do they take a second to laugh after everyone else has started laughing?
Note: Be careful. There are certain disorders among humans that can cause these signs. People with autism or asperger syndrome can often behave much as I just described. So be aware of this. However, do not dismiss someone completely as an alien if they reveal to have a disorder like this. It could, in fact, be a perfect cover for an alien not completely confident in its ability to blend in socially.
They may give things away in conversation.
When conversing with someone you suspect to be an extraterrestrial, try to get them to admit something. Don't do it so obviously that they see what you're doing, and if they do slip, act as though you don't notice. There are several questions you can ask that might cause a momentary slip in the alien's disguise if you catch them at the right time. For example: "What planets do you like?" "Do you think it is possible to travel faster than the speed of light?" "So... Roswell was pretty weird, huh?" "Can you help me with this extremely advanced physics homework I have? Specifically, I need to calculate the best route to take from Alpha Centauri to Sirius B, taking into account... um... everything."
These three tips should get you started. Remember: Trust No One. Do not inform your friends or family of your suspicions of a suspected alien. They may sell you out to win favor with the alien race. Once you are certain of the extraterrestrial nature of the person, inform the CIA. If you do not have a direct line to the CIA, I suggest calling random flower shops or bakeries and informing them of your situation. Chances are that one of them is a front. They'll get the message.
Be safe and have fun!