Listen to the song first. Becoming familiar with the song with help you tremendously with counting bars.
Look for vertical lines within the lyrics. These are bars, which divide the music into sets of beats. Some lyric sheets have the bars marked. If the lyrics are located on a piece of sheet music, you should see the bars dividing the notes. If the bars are visible, you can simply count the visible bars.
Look for the time signature of the song. This is a pair of numbers, with one on top of another, such as a "3" on top of a "4" or a "4" on top of a "4." The top number indicates how many beats are in each measure. A measure is the space between two bars.
Listen for the pattern of beats indicated by the time signature, or if there is no time signature, pat the beat of the song as you sing or say the lyrics.
Notice the repeating pattern of beats. For instance, in many songs you can hear a repeated, "1-2-3-4" pattern. Each time you tap this pattern while singing the lyrics, you have marked one bar.