Finding the Simulator
You can test with the iOS Simulator that comes with Xcode. Navigate to the Xcode app in the Finder, right click and select "Show Package Contents":
Then navigate to Contents > Applications, and open the shortcut to "iPhone Simulator" (it may be called "iOS Simulator" depending on which version of Xcode you're running):
Viewing & Testing
When you're in the simulator, click open Safari, click on the URL bar, and you can paste the testing URL using ^⌘V (Edit > Paste Text), then hit enter.
You can test different devices and orientations using the Hardware menu:
You can use Safari's inspector to debug web pages running in the simulator. Just open Safari (in OS X) alongside the simulator running Safari in iOS, and go to Develop > iPhone Simulator > [your device/tab]: