You have certainly put your finger on the hardest part of researching ancestors who may have been involved in the Revolutionary War - either in active-duty service or as a patriot.
One thing to focus on is that when seeking to join DAR, only direct-line ancestors are relevant.
John Yarnall 1739-1799 could not be son or parent of John Yarnall 1725-1809. So unless you can prove that John who d. 1799 was a direct ancestor, he would not be relevant to your DAR project. If you develop your family genealogy further, you might eventually learn that the two Johns are distant or close cousins - or no discoverable relation at all. But that would not be part of your DAR quest.
There were three levels of military organization during the Rev. War. The Continental Army was authorized and paid by Congress; State Troops were authorized and paid by the States (some units were folded into the Continentals); and County Militia.
The Militia were composed of all free white able-bodied males aged roughly 16-60 (varies by time and place) who resided within a given County. The Militia was organized for defense within the County, although a very few militia units (particularly on the frontiers) were drafted to go on emergency or major expeditions outside their County boundaries. Most militiamen did not see active duty, but among the active-duty possibilities were patrolling, guarding prisoners, or assisting escort of supplies bound elsewhere. The militiamen were required to turn out for regular drills; many rosters of those reporting survive, but reporting for such musters is not considered active duty.
One resource for determining if your John had a record in the militia of his home County, is in the Penna. Archives' website here (if you have not already looked at it):http://tinyurl.com/3nuywxh
You can click on the County in which your ancestor lived, and get an idea of battalion names and commanding officers. Then click on the "Military Records" link to get to a page linking ARIAS, which includes images of a card file of extracts from items held by the Archives plus from the published _Pennsylvania Archives_; it is arranged alphabetically.
Locally held militia lists may still exist in County records and in local Historical Society manuscript collections. The County Court records might mention a petition by a wife for subsistence support from the County while her husband was away at war. Where there was more than one person by the same name in the same County, such records help to distinguish between them.
If you wish to join DAR, you must be nominated by a chapter. Most chapters have someone assigned to assist a prospective member to prove the ancestor's service and to prove the applicant's lineage. The DAR site you visited has a list of chapters under the Member tab, arranged geographically. You might contact a chapter near you, or one near where your ancestor lived.