To measure the angle of something, you simply have to select the angle tool in the toolbar menu. Draw your first line on part of the angle you want to measure. You can lift up your finger to draw the other line. Then start your next line where you ended your first one.
For instance, if you are trying to measure the angle between, foot, knee, and hip, draw your first line from the foot to the knee. Start your second line at the knee and end at the hip.
Watch this video to see the angle tool put to use: