If you are singing in falsetto, you will notice that you won't need to exhale as forcefully to sing high notes. You should also sense a relaxation in the muscles that control your vocal chords. You will also be able to sing high notes much more quietly when singing in falsetto.
There is a distinct and recognizable physiological shift in the mechanics of the vocalization process when we switch to falsetto. Most people can feel this shift as it takes place. This altered process of vocalization will also create a different tone.
In order to sing higher notes, it is necessary to lengthen and stretch the vocal chords (like tightening a guitar string). The tighter vocal folds will vibrate faster and thereby produce a higher sound frequency. Every singer will reach a point where their vocal chords are at maximum length (stretched to the max).
But alas, there is a way to eke out a few more "false" higher notes by switching to falsetto. Falsetto allows us to sing higher notes because the action of the vocal chords is altered in a way that allows a different part of the vocal anatomy to vibrate at a faster rate than the maxed out main vocal folds.
When we switch to falsetto, the vocalis muscle relaxes allowing the cricothyroid muscles to exert more tension on the vocal ligaments. During the use of falsetto voice, the main vocal membranes are relaxed and a faster vibration is permitted to occur in the vocal ligament which has now been allowed to stretch tighter than it could before it was released by relaxation of the vocalis muscle.
This relaxation of the vocalis muscle also creates a situation where the vocal folds are not drawn together. Therefore air can more easily and freely pass across the vocal chords and vocal ligaments because you don't need the air pressure normally required to push through the vocal folds. Since less air pressure is required, you can sing high notes in falsetto at lower volumes.
For most male singers, falsetto will sound breathier, not as loud, not as present, not as dynamic, and not as rich. Although there are a few exceptional male singers who can deliver quite a rich and robust sound using falsetto, most will sound a little thinner in falsetto.
It is said that with practice, one can blend some falsetto with head voice and learn to transition more smoothly. But for most male singers, there will be a discernable loss in the richness of the tone when singing in falsetto verses head or chest voice.
Here is a short YouTube video that illustrates the shift into falsetto in a male voice. YouTube Video - male falsetto
The Beatles used falsetto often in many of their popular songs. In this video you can hear Paul McCartney switching back and forth between head voice and falsetto between 3:20 and 3:38. Also the background vocals are being sung in male falsetto. Paul McCartney in Let It Be on YouTube
MORE EXAMPLES MALE SINGER SWITCHING INTO FALSETTO:
John Mayer Very clear example of switch into falsetto at 3:22
Elton John Tiny Dancer In the chorus he switches in and out of falsetto to hit the highest notes. Watch starting at 2:24 through the chorus.
Elton John Rocket Man Several examples appear in the verses and chorus. Listen at 2:09-2:13 and again at 2:37-2:41 as he switches into falsetto to hit the highest note. Again at the end of the song he uses falsetto on "long long time" to hit the highest note.
Billy Joel Leave A Tender Moment Alone In this song Billy Joel continually goes in and out of falsetto to hit the highest notes. Prime example from 1:29-1:44
Cover of Marvin Gaye Hear great example of switch to falsetto at 5:03 - 5:09