Service choreographies specify the intended interaction protocol among a set of cooperating services at the business application level. For end-users the non-functional properties exposed by a choreographed service composition can be as important as its functional behaviour, if not even more. Therefore, in any choreography development process, the capability of specifying and assessing the established Service Level Agreements (SLAs) becomes a crucial requisite. However, by their very nature, choreography requirements can be quite abstract and may on purpose avoid formalizing non-functional properties for every step of each individual service, nonetheless the overall QoS choreography will be affected by them. In this paper, we propose a monitor enhanced with the capability to detect potential deviations from a choreography-prescribed QoS level, based on the observed non-functional behaviour of the contributing services. Such an apprehensive monitor, as we call it, can thus contribute to predict SLA violations in due time for taking useful counter-measures, and not only detect them after they have occurred. We illustrate the feasibility of the approach on a use-case from the European Project CHOReOS.