Here’s how to know… The first is an obvious one, give them a call or their next of kin. Try and find out where they are, why they haven’t called and if they are actually okay.
It might come to light the employee was dealing with a personal matter such as their partner being in labour or hospitalisation following a serious car accident. In these cases, we would all pretty much agree, these are satisfactory explanations for why the employee hadn’t contacted their manager.
However, if you’re still calling out “Bueller? Bueller?”, then your next bet should be a formalised one. Send a letter informing the employee of the situation and request them to make contact within a certain time frame, who to contact and by when, as well as advising the employee that failure to respond may be interpreted as their abandonment of employment.