How can I determine if a typed document, signed by all parties, is legally binding and holds every party to the partnership agreement?

In a few words, you can't. Specifically, you can't tell whether the document has been amended subsequent to execution, you can't tell whether all of the current parties have signed it, you can't tell whether all of the signatures are valid, you can't tell whether all of the signatories were authorized to sign and not under some kind of disability (such as infancy), and you can't tell whether the terms, conditions and provisions of the agreement are themselves lawful and enforceable. An attorney may be able to assist you in getting closer to the truth.