It's helpful to verify and fine more details on the information presented in markers. The more detail you can find, the more likely your content will be favored to post, and chances are the more internet traffic your post will get.
These are some sites I have been using to fact check, verify locations, and do general research. Please keep in mind that while we allow 1-2 sentence excerpts of copyrighted material, if the source is properly cited, we do not tolerate plagiarism. We allow excerpts of any size from public domain materials, and prefer those sources be cited as well.
Historic Map Works - A wonderful repository of old maps, drawings, and panoramas. It's a commercial business that sells prints, but you can browse everything for free.
New York Roots - A nice collection of articles, cemetery listings, and reprints of old books.
The Internet Archive |Texts - Public domain books. All searchable.
Open Library - Public domain books. All searchable.
Google Books - Public domain and commercial books. All searchable.
Rootsweb - A lot of this, especially the "websites" section, is user generated content. Be weary of the accuracy of information coming from parts of this website. Aside from that, it's a great way to verify names, dates, and birthplaces.