Which of the following elements has the smallest numerical electron affinity: B, C, N, O, or F?
You just need to familiarize yourself with the periodic trends. There really is no shortcut for this.
Electron affinity can be defined as the capability of an atom to gain an electron. The trend for this is directly opposite from electron negativity (which is an atom's capability to give away electrons).
In other words,
Electron affinity = electron poor
Electron negative = electron rich
The trend of electronegativity tends to increase from left to right of the periodic table, and from top to bottom (of the periodic table); which means whatever trend opposite of that is your trend for electron affinity (that is right to left, bottom to top).
Hope what I said made sense to you. :)