You can judge a player's overall strength by the ATK number on the main screen, and the strength of weapon and eidolon grids by the ATK number on the respective screens.

Generally, what I want to know when someone is asking for advice is the following:
Do they have a +100% eidolon?
Kamihime, especially SSRs, that they have access to. This is usually the long-term bottleneck on progression.
Do they have typical debuffs covered with the kamihime, Soul, and EX ability they've selected?
Are they using the best primary and friend eidolons they have access to? Do they understand the difference between character and elemental attack?
Have they leveled up their assault skill for the element they're using?

The attack power is something I don't even necessarily pay attention to, because if they're doing all the rest correctly, that will come with time. The rest is probably only necessary if a problem is suspected and needs to be troubleshooted.