I am trying to implement the following resonant oscillator for wireless power transfer application. There will be a secondary coil coupled with Lp. 
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Reference – WPT Application Note
One of the pitfalls of this circuit is that the power consumption can be high when the secondary coil is absent.
Is there a way to design an auxiliary circuit to detect when the receiver is not present and to reduce the idle power consumption (possibly, by introducing a high series impedance and decrease resonant current)? However, it should also be able to detect when the receiver is coupled and activate the normal operation.
Can we use something like tickler coil for this purpose?
Edit:
I want to exclude obvious approaches which require the installation of additional sensing layer, for example, proximity sensors to detect receiver or additional communication link.
Next, as some of the answers suggested, using a simple micro to detect the changes in primary phase or define a current threshold to cut off the supply can be implemented. However, such approaches are highly dependent on the circuit parameters (frequency, power, etc) and most importantly, these sensing become challenging if we want coupling to be weak. When coupling is small, reflected impedance may be very small to make significant changes in primary side.
Best Answer
I think the first step to a circuit like this would be to measure the current through the Lp coil. A current measurement typically consists of a low value resistor and a current amp. The current would change if a receiving coil were placed near the transmitting coil. A threshold could be set on the current measurement (with a comparator)to allow the circuit to determine if the coil were there or not.
The circuit could be turned on periodically to determine if there was a device there and go into high power mode.
Another way the detection of a coil could happen is by monitoring the phase of the current and voltage phase, the receiving coil would change the relationship between them, this is how it is done on some wireless power IC's.
You don't have to do this with a microprocessor, you could use a 555 timer for the pulse to check the circuit, after you did so you could 'and' the result together with the current detection signal, if it's high then keep the power active, if it's low, then shut the circuit down.