Mycroft, to be fair, if you try to discuss, you'll find discussion. I don't think you were interested in discussion.
For OTC markets there isn't really a price and a deviation from it. There is some sort of consensus, and in Forex (spot) I've found that for what I use, the difference is rarely more than a pip between different brokers. Besides there is a bid and an ask.
For exchange traded products, you compare with the exchange.