We consider the problem of detecting and mitigating the effect of multipath on GNSS observations. In particular, we focus on carrier-phase observations that are collected at an array of GNSS antennas. We exploit a special antenna array geometry, synchronization among the GNSS receivers, and an attitude estimate provided by an attitude filter or an IMU, to develop a technique to identify the satellite observations that are contaminated with multipath. The proposed technique leverages the antenna geometry to rabidly estimate the attitude parameters for various satellite combinations. Next, a dedicated decision-making algorithm is used to identify the satellite observations affected by multipath. Our simulation results demonstrate the effectiveness of the proposed approach in detecting the occurrence of multipath with high success rates. By rejecting the multipath-affected observations, we show remarkable performance gains when attitude determination is considered as an example application.