Question

I zip string to byte array and unzip byte array to string in my .NET application.

Is it possible to check if byte array contains zipped string?
I.e. I want to avoid double zipping of already zipped string or trying to unzip plain text.

Answer 1

I could be wrong, but if I remember correctly the first two bytes of a zip file/stream have a signature. They should always be PK.
You could easily check the first two bytes of your byte array and see if they contains the ASCII values 80 (P) and 75 (K)

So supposing your byte array is called data

if(data[0] != 80 && data[1] != 75)
    // Is not already zipped