How do you evaluate how good a unit is/can be?

I'm pretty new to fire emblem, having only played awakening for about 100 hours and now just 30 hours into fates. (Still not past chapter 11 on any of my saves, keep restarting =P)

I see a lot of posts and such about evaluation of units, and without much knowledge I take them as gospel. But one description confused me. Someone mentioned that Hana's offense becomes overkill at a point, and it made me wonder just how people are evaluating characters.

How do you know how a unit's damage/defense stacks up with enemies of around the same level? It's one thing to compare player units side by side to find out which is better in which areas, but how do you know how they'll perform at any given time depending on how leveled they are?

Kind of a weird question I know, I just really liked my Donnel in Awakening and without someone similar to sweep me off my feet (Thanks Mozu -.-) I'm struggling to find out which unit/s I want to focus on because I don't know what's going into the evaluations I'm reading.