Each post office has tracking numbers like a zip code. Everything I have ever shipped from my local post office started with the exact same numbers. I could easily find a package in progress by going to the USPS web site and trying some numbers - especially if I just shipped something to anyone. I'd know what the current range of numbers is.

Otherwise one of three things will happen. Either the number won't be found, the package will have already been delivered (with a date which means I can adjust my guess) or it's in progress.

There's probably a faster, easier way to do it if you're an actual scammer who knows the system better.