The only type of wave that is truely a groundswell is a earthquake generated wave. This is not something that you typicaly see at your local beach. This is the most misunderstood factor in wave definition. Basically, this is a measure of the speed at which the wave is travelling. It is easy to judge the height of an approaching wave.
Most people estimate wave heights based on their relative size towards a riding surfer.
Simply measure the the time it takes for the top of one wave to pass till the top of the next arrives. Most periods are measured in seconds. A typical windswell will have a period of 5 to 9 seconds. Whereas, a groundswell will have a much greater period from 10 upwards of 20 seconds. From a practical standpoint, a groundswell is created by a storm far away from the beach where its generated swell is breaking.