Do USB power supplies send signals indicating energy level being delivered?

Do USB power supplies send signals to tell the connected device (tablet/phone/camera) how much power is being supplied?

Or, is measuring it the only way for the device to know how much energy is being supplied?

Does the absence/presence of such signals vary at all between different USB standards?

USB (Universal Serial Bus) is a communications bus standard that is capable of supplying both data and power over a single cable (a cable of which that can consist of many wires). Devices that come with a USB charger (or wall plugs that supply USB power) only make use of the power lines on the USB cable. The USB standards specify the current (Amps), the voltage and the resulting total power consumption (current*volts).

To answer directly:

Do USB power supplies send signals to tell the tablet / phone / camera how much power is being supplied?

No; first, there's no such thing as a USB power supply. There are devices that convert AC power into DC power and that allow USB devices to be plugged into them and thus delivering USB power to said device; like a phone's wall charger, or USB wall plugs, they have AC to DC conversion components in them that modulate the current to flow at the USB rated specs (usually 5V@1A for most of those types of wall plugs). Secondly, it's the device (a.k.a. the electrical load) that determines how much power is consumed and the power supply doesn't send any 'signals' that alert the device of how much power it 'has' beyond the electrical signal itself (i.e. AC/DC current). Getting into the details of how that all works would be beyond the scope of this question/answer and forum but if you're curious about electrical flow/impedence/etc., I recommend a Wiki click fest.

Or is measuring it the only way for the device to know how much energy is being supplied?

Yes and no. See answer to first question, but essentially, the device manufacturer builds the device with electrical components that measure how much electricity is flowing over a certain component over a certain period of time and use some EE math to determine "how much" electricity is being used (similar to the idea of an Ohmmeter). But these kinds of measurements really only apply if the device is monitoring it's battery usage or to make sure it doesn't get an over/under current to avoid damage to itself (or for overclocking/tweaking purposes).

Does the absence/presence of such signals vary between different USB standards?

Again, see first answer, but the only thing that varies between the USB standards with regards to power is the limits, that is, USB 3.x allow for a higher power consumption by the device plugged in, so if you plug in a 3.x device to a 2.0 hub, the device might not power on because it might require 900mA while the 2.0 hub can only give 500mA.

Hope that can help clear some things up.