Interestingly you can ask whether a string is ASCII, "this".is_ascii() is a predicate which does exactly that, but for what I think is your question, "are they all ASCII caps?" you'd need to go via an iterator and a lambda: "THIS".bytes().all(|b| b.is_ascii_uppercase())
The iterator is because there is no provision of each such predicate over the whole string, which seems reasonable (the character class predicates are provided on u8 (ASCII only), and on char (all of them)), but the lambda is due to Rust's 1.0 compatibility commitment plus an unfortunate historical choice. If not for compatibility we'd fix it so you could just write "THIS".bytes().all(u8::is_ascii_uppercase)
That's a heavy lift though, you may need to carry this special code around in every compiler, forever, and I haven't even figured out what such code should look like, it might be really hard to even write it.
IMHO there the logic behind that is very clear and makes a lot of sense. I think it's natural that view-related traits (the ones who operate on the elements of a contiguous area of memory) should be associated with slices, while memory-related ones (the ones that operate on an owning container) should be implemented on the owning types. If you mix them up, you are bound to mess up first and foremost because _it is not logically sound_. You don't need to own a string in order to check if all of its ASCII characters are all caps, for instance, because it's an operation that's also valid on any array of bytes, without the whole concept of "owning memory".