As posted above by the others here, exchanges sites may vary  in terms of bitcoin price value.

So for a trader, they are watching the price obviously on the exchange site where they are doing trades. So if you have trading funds on action then monitor the price in your specific exchange site. Or in other cases, you have to convert your bitcoin into cash, check the price on that exchanger.

For instant result, you can just type "btc to usd" in the google search box. The result came from the first site of the result.