Circuits Question

Well the first thing you need to know is the bandwidth of the signals you will be monitoring. You also should be well versed in linear circuit design, too. Lastly I should introduce you to an old friend.

Ask the postman!

What does Kevin Costner have to do with this?

Might wanna ask this the postman too!

Your question is so broad as to be meaningless.
You need to define what sort of Envelope as it has several meanings in a electronic/engineering/domestic context.
Then we can ask about detecting it once we know what it is.
If you mean a letter envelope that postmen deliver, it can be detected by a myriad of ways, which begs the question why do you want do detect it?

E.G a light pressure swith under your letter box could do the job.

If you write a sensible question, you will get sensible answers.
Del

is that an order? it damn sure wasn't a request

Considering that this is the communications & Electronics page I will assume that your rude question is about the envelope detector that is an electronic circuit that takes a high frequency signal as input and provides a dc output of the envelope of the original signal. Usually the purpose of the input frequency is to provide a carrier for audio frequency modulation that provides an undulating dc output for amplification of your favorite music or talk show. Redfred provided the circuit design.