Efficiently checking the initial contents of a file

[Nordlöw]

How can I efficiently using pure emacs-lisp (without calling any
external process) investigate the first bytes of a file?

My guess is
- Open parts of the file into a buffer or string.
- Alt 1. Switch to the buffer and do things.
  Alt 2. Or do stuff directly the string?

Which operations should be performed via a buffer and which should
operate on a string?

/Nordlöw