A glance at the inscription around the front face of the lens will tell you the focal length and the maximum f, or aperture number.

Portraits of people, however, are best done with more light (eg aperture numbers around f/2.8) as this blurs the background, making the person 'pop out', so to speak.

Lower aperture numbers (such as 2.8 or 2.4) translate to larger openings that allow the camera to capture pictures in darker settings before requiring the harsh light of a flash.