Considerations when Evaluating the Validity of a U.S. Patent:
1. Is the patent expired? In the United States, utility patents are enforceable for a period of twenty (20) years from the filing date, if the maintenance fees have been paid. Design patents are enforceable for 14 years after the grant of the patent. Utility patents require three (3) maintenance fees to be paid to prevent the patent from being abandoned, at periods of 3.5, 7.5, and 11.5 years after issuance. If the fees are not paid, then the patent is abandoned, and no longer enforceable.
2. Have the maintenance fees been paid? An inquiry with the proper division of the United States Patent and Trademark Office will allow you to determine if the maintenance fees have been paid. If not, then the patent is abandoned, and not enforceable.
3. Is the patent invalid under 35 United States Code, section 102? For example, this code prevents a patent from being valid if:
(a) the invention was known or used by others in this country, or patented or described in a printed publication in this or a foreign country, before the invention thereof by the applicant for patent;
(b) the invention was patented or described in a printed publication in this or a foreign country or in public use or on sale in this country, more than one year prior to the date of the application for patent in the United States;
(c) he has abandoned the invention, or
(d) the invention was first patented or caused to be patented, or was the subject of an inventor's certificate, by the applicant or his legal representatives or assigns in a foreign country prior to the date of the application for patent in this country on an application for patent or inventor's certificate filed more than twelve months before the filing of the application in the United States.;
(e) the invention was described in - (1) an application for patent, published under section 122(b), by another filed in the United States before the invention by the applicant for patent or (2) a patent granted on an application for patent by another filed in the United States before the invention by the applicant for patent, except that an international application filed under the treaty defined in section 351 (a) shall have the effects for the purposes of this subsection of an application filed in the United States only if the international application designated the United States and was published under Article 21(2) of such treaty in the English language...
The above statutes has a few other provisions that may cause a patent to be invalid.
These considerations are only a starting point and a proper evaluation many times requires a thorough review of the applicant's filings, and office actions from U.S. Patent and Trademark Office .