If there is a difference between curiosity and nosiness, then I am the former. This means if you leave me alone with your mail, I will glance at the pile, but I won't sift through it and analyze each item. I most certainly won't open anything because, well, then I'd be totally busted. You can tell a lot about your beloved just by the envelope.
Only really personal things come in colored envelopes. Your utility company isn't going to make the bill less painful by putting it in a pink package, so they don't even bother. Cards, love letters and invitations come in pretty envelopes. If you see any of these in the mail pile and are concerned say, "Oh, look, honey! You got an invitation! Open it and see if we're free that day." If it is an invitation or something even more innocuous -- like a greeting from Mom -- then your lover will gladly open it in front of you.
If something like, "It's nothing. I'll look at it later," is said, your antenna should go up.
Like the government, lawyers love to send things by mail. If it is something really serious you don't even have to look at the return address, just peep out the green fringe from the signature card the recipient had to sign. Believe me, as someone who has received way too many letters from lawyers, the letters you have to sign for mean something serious. And not serious like you've inherited money from a long lost relative. It means lawsuits and legal troubles. If you see something like that, ask about it. The comfort you'll get from the knowledge outweighs the berating you'll get for being nosy.
Other mail isn't as telling but there is something there. Lots of letters from charities means your lover at some time gave money to a charity and ended up on a mailing list. I like people who give to charities, so this is a good thing.
Lots of catalogs is also the result of ending up on a mailing list, but it's just as telling. My requests from charities are all from animal rights organizations. Likewise my catalogs are all from clothing stores and Disney. If your lover is getting the latest bondage and domination gear catalog, it's not by accident.
Bills really don't say anything about a person, so if you see lots of envelopes from credit cards it means nothing. I get notices reminding me that I have a zero balance. Don't look for trouble where there isn't any. However, if you see one stamped "Past due. Pay or we'll kidnap your dog," it's time to give your relationship some thought.
Finally, look at the magazine titles. This is the only mail you receive that's totally your choice. If your lover is receiving something you find offensive and she says, "It was a gift. There's only three more issues coming," she's lying! If the magazine is wrapped in brown paper and you can't even tell what it is ... you're on your own on that one.
Like many people I throw my mail on a table until I'm ready to look at it. So I'm certainly vulnerable to prying eyes. Keep in mind that you may be judged by that. Of course, if you catch your lover, feel free to grab a bat. After all, reading someone else's mail is a federal offense.