You could google the address to see if it gets any hits, but if it is off market, then best is to contact local agents, signup to their mailing lists, and ask them any off market sales. Some of these sales are because seller doesnt want it advertised they are selling, maybe being forced to sell, but hard to know whether these are sold to friends of the agent, as if not advertised who is to say what is the max it could sell for, so maybe seller gives expectation and if that reached then sold.
Maybe someone who has sold/bought off market could shed some light on the process, as if no other parties aware its for sale, could bag a bargain.