How do I read through a sealed envelope? Tricks?

maybe you can steam open the envelope

Second the steaming. I think the classic way is to put some water and the envelope in the microwave? But don't quote me on that

Also noting that tampering with other people's mail is against the law, yadda yadda

Wow, sorry I missed this one. What was in the envelope?

i think the actual "classic" way probably doesn't involve a microwave... try ironing the letter with a lot of steam. but of course *opening* a sealed envelope is very easy. reclosing it once you've opened it is the real trick. consider the attentiveness of your audience. will the person whose mail you're reading notice if you open really carefully and then re-seal with a glue stick? probably no...

If it’s not a heavy or security-print envelope, throw it on a backlit scanner (any flatbed scanner with a nice, bright transparency lamp should work) and get a 24-bit color scan. Open the scan in Photoshop and manipulate the gamma correction or color correction curves to maximize the contrast between the envelope and the text inside. Rotate as necessary. You can usually get something and you don’t even have to break the seal. This, of course, is unlawful and unethical, and should never actually be done.

use a Standard Issue Federation Tricorder. Some helpful Starship commander might even let you use his sensor array.

Put it under a light- lets you see everything :D