There is something called an RSR Report that comes out every month. You'll get an email from SAPA with it usually between the 10th and the 20th of each month. It'll show you the "targeted lines", aka how many lines they anticipate to have for the next month, at each base. Compare that with the base-specific seniority list, and where you would fall on that list given your seniority number. If you're below the target, you'll likely be able to get a line. Above it, reserve. Once you get access to PBS, you'll be able to get a more detailed look on how you stack up against others within your current base, and do some estimations on how many people are leaving per month, and how many months it might take to get within the targeted lineholders.

This is how I've been looking at it, idk someone else might have a more efficient way.