The USB port has 4 pins. Normally, pins 1 and 4 are power/ground, and pins 2 and 3 are differential data.
The USB device will signal speed to the host/hub by using a pull-up resistor on one of the D- and D+ lines.
A device that charges through USB will not draw more than 500 mA when just connected, because this is the maximum allowed by the USB bus. In fact, to be conformant, the device should not draw more than 100 mA without negotiating with the host -- thus, there needs to be logic on the host side (that supplies power) to go to 500 mA.
Because plug-in chargers shouldn't need to be USB hosts, a standard was developed where the charger can short D- and D+ together. The device can then detect this using its pull-up resistors and some logic in the device, and this is a signal that the device can draw 1A.
No standard exists to signal that a device can draw more than 1A, so most phones do not. There are some special cases with USB 3.0, and with iPhones that use some resistance for the shorting circuitry, which are not well documented. Unless you are charging an iDevice, or can provide a USB 3.0 host, don't worry about this.