How to screenshot snapchats without them knowing?

You have 2 options.

1) root your phone, get the Casper snap chat client.

2) get a program called Blue Stacks for windows and install the Casper client on that. (Blue Stacks is an android emulator)

Either way, though it's pretty common for your snap chat account to get locked for 24 hours for using it though ;_; (Until the dev figures a way around snap chat detecting you using Casper)