Clocks on wireless sensor nodes experience a natural drift. This clock skew is unique for each node as it depends on the clocks manufacturing characteristics. Clock skew can be used as unique node identifier which is, among other applications, useful for node authentication. We describe how clock skew of a node’s clock can be measured directly on a node by utilising the available high precision radio transceiver clock. We detail an implementation of this proposed local clock skew tracking method for the Zolertia Z1 platform. We determine the required sampling effort to accurately determine clock skew. We also discuss how clock skew measurements can be aligned with existing transceiver operations in order to avoid an increase in energy consumption.