That's a broad question, but if you are familiar with reading excel charts, here's one with some initial information that could start you off.

If it's just generally understanding seniority, one of the biggest things to understand for a lot of seniority related questions (should I move in base, should I bid up right now or wait, should I commute to EWR or stay in DC) is to know the distribution of seniority on your desired BES, rather than just the "foot in the door" number of the junior guy.

For example, LAX is thought to be a junior base for many because it's on every new hire bid, but much of the LA pilot pool has been stagnant for a while. Don't bite off on the idea that a low junior man means the base is junior.

Using a rough estimate of 400 retirements per year, you can also gauge how your seniority equates in years with the company. In many parts of the distribution, picking EWR over SFO puts you about 2 working years of seniority less than you'd have otherwise, but there are so many caveats. Then again, in other parts of the curve, EWR is more senior.

Depending on why you are looking into it, you also have to be aware of what's driving your seniority with the people +/-500 of you. Hiring started in droves in 2014/15, and there was a relative trough in 2017.
So if you were a 2017 hire, there's a large bubble of people about you that skew expectations.

There are some things that someone can draw though that are more solid, not that you wouldn't know the information from guessing though. EWR737CA has a very sharp G-line dip, a lot than SFO. So if you were wondering about bidding CA slightly below the G-line and figuring you'd get a line in a few months, you'd probably be right in SFO, and probably would be on reserve for a while still in EWR depending how future bids go. It seems that for some reason, people tend to wait until they'll have a line in EWR more.

Oddly enough, there isn't as a pronounced G-line dip in SFO WB as I thought there'd be. The entry into those fleets has a sporadic path. I think I know why - it seems the line likely corresponds to vacancies in those fleets having been infrequent and inconsistent, so people have been taking those bids when they can, rather than thinking, "I'll just get it when I'm about the g-line".

Finally, past performance doesn't equal future results, so this is really all garbage if the pilots in the future don't behave in the same way on bids than the pilots did in the past, which is pretty much the case.