It depends upon whether the person is alive or dead, which war, which military service, and which country.
For the United States:
For yourself AND for people who are dead, the National Personnel Records Center (NPRC) is the place to go. Here is the information you need:
To request military service records held by the NPRC, veterans and the next-of-kin of deceased veterans can use their web interface at vetrecs.archives.gov. Or they and others can use the Standard Form (SF) 180, Request Pertaining to Military Records. Although not mandatory, the SF 180 is the recommended method to send a request for military service information. This form captures all the necessary information to locate a record. Provide as much information on the form as possible and send copies of any service documents that you may have. Requests may also be submitted as a letter. Your letter should parallel the basic information requested in the SF 180. Please see that form for details on the information that you should include within your letter. Follow the instructions for preparing the SF 180. Check the table on the last page to determine the location of the record and submit your request to the appropriate address. In most cases, you can * Send by Mail The NPRC's mailing address is: National Personnel Records Center
Military Personnel Records
9700 Page Avenue
St. Louis, MO 63132-5100
* Send by Fax Their fax number for requesting military records is 314-801-9195. Although fax IS faster, the NPRC will respond in writing only by U.S. snailmail, and that would include any problems that they may encounter - all of which increases the length-of-time the NPRC takes to deliver your DD214. -----------------------------
Now, as to verify that the person "served in a war", here are some tips:
1. If you can obtain the DD Form 214 or DD Form 215, you can read it and it will give details about "Wartime" Service. You may have to get another Veteran to explain some of the abbreviations to you.
2. You can go to any number of Veteran's Web Sites, and plug in a person's name. There are many web sites which are maintained by military organizations which allow you to look up a person's name and service. If the person claims to have been a member of a unit, for example, the 7th Radio Research Field Station (7th RRFS), you can go to their web site and see if the person is listed. Just because he or she is NOT listed doesn't mean that he or she did not serve, it only means that his or her name is not listed. There are no guarantees.
3. If the person claims to hold a high decoration, such as the Medal of Honor, The Distinguished Service Cross, the Purple Heart, the Combat Infantryman's Badge, etc., or if they claim to be a member of "Special Forces" or status as a Prisoner of War, you can go to those web sites and make an inquiry.
4. You can go to a local military organization such as the Veterans of Foreign Wars (VFW) or the Military Order of the Purple Heart (MOPH) and ask your question of their membership chairman.
5. If you are researching your family history and believe that an ancestor served in a past war, the best place it one of the Genealogy web sites. They require a fee but are very accurate. Also, the Church of Latterday Saints in Utah has just about every bit of family tree information you can imagine so you can Google for the LDS Family Tree sites and do your checking there. All wars from the Revolutionary forward to the various Banana Republic Wars of the early 1900's are available at various family tree sites. You just have to Google for it. Say the person is your great grandfather and served in the Spanish-American War of 1898 .. just look for one of the many sites and find the personnel lists for various regiments and so forth.
6. Last but certainly not least, be sure to be specific in your inquiries. There is a lot of difference between "a tour of duty in Honduras" since the person could have been there with Task Force Bayonet in the 1980's or during the Banana Wars of the Nineteen Hundreds.
7. Remember, also, that if you are going to use this information to "challenge" someone who claims to have been in a war but who you think is not telling the truth, it is always better to obtain assistance from knowledgeable former or retired members of the military who know how to deal with "Wannabe Veterans".