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We consider a sheared granular system experiencing intermittent dynamics of stick-slip type via discrete element simulations. The considered setup consists of a two-dimensional system of soft frictional particles sandwiched between solid walls, one of which is exposed to a shearing force. The slip events are detected using stochastic state space models applied to various measures describing the system. We show that the measures describing the forces between the particles provide earlier detection of an upcoming slip event than the measures based solely on the wall movement. By comparing the detection times obtained from the considered measures, we observe that a typical slip event starts with a local change in the force network. However, some local changes do not spread globally over the force network. For the changes that become global, we find a sharp critical value for their size. If the size of a global change exceeds the critical value, then it triggers a slip event; if it does not, then a much weaker micro-slip follows. Quantification of the changes in the force network is made possible by formulating clear and precise measures describing their static and dynamic properties.