What do you mean by "identify an old shipping label" ?
The tracking number should be available in the PayPal transaction(if the sale was a PayPal transaction and you uploaded it or it was automatically uploaded), and USPS tracking should be available for the tracking number for 120 days.
eBay shipping label management page
http://my.ebay.com/ws/eBayISAPI.dll?MyEbayBeta&CurrentPage=LabelManagement shows label info for 90 days. Tracking number, from & to addresses, cost, mailing date, etc.
If you are looking for the actual label itself for whatever reason, that will be available for 24 hours (for eBay labels printed labels) via that shipping label link or "reprint label" function associated with an eBay item. Same idea for PayPal generated labels - 24 hours to reprint via the PayPal transaction.
After that, label itself is only available from the PDF (or whatever format) copy of it you saved for your records.
If you really meant tracking, then you should be ok less than 120 days. After 120 days, it appears it is possible to get tracking information via a USPS FOIA request. (
https://community.ebay.com/t5/Payments/Paypal-INR-case-opened-after-4-months-tracking-no-longer-in-U... )