Checking if a file is binary (non-textual)

[Nordlöw]

What characters (bytes) should *not* be present in a text-file that
may contain variable-length unicode characters.
What does the unicode standard say about this?

The reason for asking:
I am working on a tool that unifies grep, tags-query-replace, occur,
etc.
And I really would like this tool to have some clever default
behaviour for determining how to present the search (grep) hit-context
for different file-types:
- textual files: show whole line (as grep and occur does)
- binary files: either no context just notify match (like grep) or
maybe all [a-zA-Z0-9_]* directly before or after hit
- ...

/Nordlöw