Unfortunately I've never gotten that sense from a letter, doesn't help that the vast vast majority of what I deliver is junk people are just going to throw away anyway. I also deliver in a town that is primarily a public school and retirement-age folks. I have seen what were fairly obviously letters to Santa, or otherwise kid's letters to some other family written all scraggly in crayon or colored pencil.

Sometimes I can tell that some parcels I have are medications, those are fairly obvious. Even had a few times where someone obviously ordered dumbbells, that was... fun. Also quite a few boxes are just the standard packaging, nothing discrete, I've always felt a little worried every time I leave a monitor or a tv on someone's doorstep hoping it doesn't get porch-thieved. I'll most often ring the doorbell or knock hard for these just in case.