One of the distinguishing marks is complete absence of dogmatism in their opinions, especially in regard to religion or politics. This is because they will have read too many different valid viewpoints, freeing them from "tunnel vision." So they will be blessed with an open mind.
Another "give away" of being well read, especially with regards to literature, is usage of idiomatic language rather than just the correct one. Each language has its own unique idiom, which brings it to life. Also use of appropriate references to support & bolster one's statements. (Also proof of being really well read!)
Even people who are not well read, now have an itch to appear so & publish their writings. Instead of writing on a subject which they love & have abundant knowledge, they start with zero knowledge or love of the subject, but start writing anyway, gathering material on the way. This type of "writing to order" just to get recognition, turns out insipid for obvious reasons, & would be recognised at once for what it is by well read persons.