The main sources to Viking presence on North American soil before Columbus' discovery of America are the Greenlanders' Saga and Erik's Saga as well as archaeological evidence at L'Anse aux Meadows in Newfoundland, which is likely Straumfjoðr, the northern camp of Vinland described in Erik's Saga. The sagas as written sources are always to be treated with caution, but the archaeological evidence does suggest that an unmistakenly Norse settlement was built on Newfoundland in the early 11th century lending the sagas some credibility in this regard. The settlement doesn't point to colonisation however as it looks like the Viking winter camps found in Europe and it was likely used as a trading port to transport materials from the Vinland settlement back to the Greenland settlements. It did not last for more than a few years as suggested by the size of the and the cultural deposit of themiddens settlement, so the Viking "discovery of America" was likely a short enterprise in total.

Source: Birgitta Wallace (2012). "The Discovery of Vinland." in The Viking World (edited by Stefan Brink & Neil Price). New York: Routledge, pages 604-612.

Edit: New Foundland is now Newfoundland.