Most of the time they are sensed with someone who is a medium, but we all have access to these skills. For instance a sensation where the hair on the back of your neck stands on end and you feel as if something is watching you. That is the most that some get when contacting spirits.. Learning to contact them and hold conversation however is a completely different story.
Open and align your chakras. The third eye is a the chakra that is a sensory pad that amplifies your senses. It will cause you to see, hear, taste, smell, and feel energy and other things that you may not have noticed before. Your third eye will also help you to gain the skill of telepathy and communication with spirits. You can also use a spirit board to communicate with those who are on the other side.
Spirits and Ghosts live on a different plane then we do, so they are probably all around us. Some people can sense them better than others, some are just kind of 'born with it.' And other times its the spirits who make connection. Some spirits will be around us but don't make any contact with us.
Theres different ways you can sense a spirit. Some people get cold or hot, some people get a tingly feeling or that 'somebody's watching me'-feeling. some people get goosebumps or their body hair can stand up. Some people feel dizzy or even sick.
I'll warn you though with connecting them. a lot of times things can go wrong quickly and you can end up with a bad spirit. not all spirits are friendly (They aren't all even though, Ether!) so you should always use caution.
If you want to connect a spirit it should be for a good reason, not just to play around. It'd probably be a good idea to keep protection charms (Herbs, Runes, pendants, crystals, etc) with you when you are trying to call on them. You should do a protection spell or exercise before you make contact.
You don't contact them unless you're naturally gifted to. Otherwise if you were meant to contact them, then you would have had the gift to do that. My suggestion is to leave them along as they're not bothering you.
I think ChaosxBliss is correct .
I believe that most people feel the spirits ,especially if they dwell in their places for long and are not just passing by .Animals also are sensitive for spiritual energy especially dog and horses .
However if you really want to communicate with them you do need some basic skills or natural talent to properly communicate with them .But you need to keep in mind not all spirits will communicate with you just because you were in playful mood .
Also you can really get yourself in trouble by attracting the wrong entities and then not being able to deal with them .So i will rather recommend that you know how to banish them and cleanse yourself and your property .