Have the person lie comfortably on a massage table or bed. Hold your hands a few inches above your friend's body, and move them slowly up his or her central channel, from a few inches below the groin to a few inches above the top of the head. See if you can feel any difference over where the chakras are.
Some people may see colors or images related to the chakras, but (to my knowledge) it's more common to feel sensations with the hands.
What people feel when they are sensing subtle energy is highly individualized. When I first started learning energy healing, mostly I could only feel a buzzing in my hands, or warmth. But as time went on and I gained more experience, my hands became more sensitive.
I can't always describe in words what it is I'm feeling when I'm sensing chakras, but the feelings are usually very definite. I want to stress that everyone interprets through different "filters," so everyone probably has different experiences. But for me, the best way I can describe it is that generally I have a sense of more "activity" over the chakras than in other areas of the body.
Sometimes I feel imbalances in the chakras, with some chakras giving me lots of "information," and others little at all. Sometimes I feel "releases" from the chakras, like little puffs or even bumps against my palms. Sometimes as I work with the chakras, I feel the quality of the energy change in intensity or quality, often increasing in what I might call richness, dimension, or integration.
But these are my sensations and my words. What you feel and how you describe it could be very different!
If you don't feel or sense anything, don't worry. Allow yourself to imagine that you are feeling something! This is a good way to unblock your perceptive abilities.
Also, know that if you think you feel something, you almost certainly are. Just trust that it's real, and know that with practice your sensitivity will grow.