You (technically) ask different questions in the title and the body. On Stack Exchange, contributions $\neq$ reputation and posts. There are many, many, many other ways in which a user can help out:
- Editing posts without making trivial edits
- Completing review tasks without merely skimming the post at hand
- Participating on meta without simply giving knee-jerk opinions
- Adding (helpful) comments without making noise
- Voting without blindly clicking a mouse
Here are some links with which you can explore the users who do each of these the most:
These are all valuable contributions. However, you can't just point to a user and think, "This guy is #$a$2 on edits, so his contributions must be really helpful!" because you have to look at the edits to judge how much they help a post (if at all). Don't be superficial.
Now, in your question body, you write
So, What other ways beside rep can gauge the quality of a user's posts,
Well, read them, as Did said! Some posts get ridiculously high amounts of upvotes when they aren't much better than average, because the question at hand becomes quite popular. The reverse can also be true: great answers can be given to questions with low views, and so they'll receive lower scores. I always read posts in their entirety before voting. Don't be a sheep and do what everyone else is doing.
Stack Exchange has the paradigm for users of "I'll use my vote however I want!" So votes aren't always the best way to measure contributions. Accepted answers are better, but remember that only one answer can be accepted per question, and answers to poor, off-topic questions aren't always worth as much as answers to good questions - unless the answers provide great assistance to the person asking the question and others viewing it in the future.
So reputation can be a good indicator of contributions, although churning out a bunch of sub-par posts can get you the same rep as a smaller amount of high-quality posts. I know this might sound odd on Mathematics, but you have to look beyond the numbers and read the posts themselves.
And look at other ways the user helps the site, though editing, reviewing, participating on meta, commenting, and voting - and some other helpful things, like being around in chat.
The rep system works pretty well, but don't judge a user purely by rep. Look deeper.
1 Sorry; I'm not too good at using Data Explorer.
2 Let's assume that a is really low.