Records? Multiple records confirming major events. For small events that are not as well known its just a matter of piecing together records, documents, newspaper articles and whatever other material you can find to confirm that your event actually happened. This is why I find history so interesting. I could collect documents and put together a story and think I have exactly what happened, and in 5, 10, or 50 years later one document could appear that could completely alter the way you look at what you thought happened, if that makes any sense.