We address the problem of checking properties of Time Transition Systems (TTS), a generalization of Time Petri Nets with data variables and priorities. We are specifically interested by time-related properties expressed using real-time specification patterns, a language inspired by properties commonly found during the analysis of reactive systems. Our verification approach is based on the use of observers in order to transform the verification of timed patterns into the verification of simpler LTL formulas. While the use of observers for model-checking timed extensions of temporal logics is fairly common, our approach is original in several ways. In addition to traditional observers based on the monitoring of places and transitions, we propose a new class of ob- servers based on the monitoring of data modifications that appears to be more efficient in practice. Moreover, we provide a formal framework to prove that observers are correct and non-intrusive, meaning that they do not affect the system under observation. Our approach has been integrated in a verification toolchain for Fiacre, a formal modeling language that can be compiled into TTS.