Super-high mechanical strength and excellent lubrication property at the nanoscale render graphene a promising ultra-thin solid lubricant. However, macroscale friction tests have shown that wear of graphene often occurs resulting in failure of interface lubrication. As graphene coatings are atomically thin, a sensitive and reliable detection method for monitoring wear has been a great challenge to date. In this work, we performed tribological experiments on monolayer graphene on SiO2/Si while simultaneously measuring the in-plane electric resistance of graphene during the sliding tests. Our experiments showed that the electric resistance exhibited a sudden increase signifying the onset of graphene wear well before the macroscale friction increased rapidly. The variation of in-plane electric resistance was found to depend on the wear track length and relative orientation. Longer wear track and wear track perpendicular to the current would typically lead to higher increase in graphene electric resistance. Our work demonstrates that monitoring in-plane electric resistance can be a simple, sensitive and predictive method for detecting graphene wear, which makes it a useful inspecting technique for a wide range of mechanical applications.