For wireless cellular communications, a “smart” multi-band combiner system has a multi-band combiner and a passive inter-modulation (PIM) detection sub-system. The multi-band combiner combines multiple transmit signals in different downlink frequency bands into a single, multi-band transmit signal for transmission from a cell tower antenna. The PIM detection sub-system characterizes the frequency components in the multi-band transmit signal to predict PIM products and determine if any predicted PIM products might interfere with any receive signals in any uplink frequency bands. If so, the PIM detection sub-system generates a signal indicating the presence of such predicted interfering PIM products, and the system installer and/or the network administrator can take remedial action to prevent the PIM products from interfering with user communications.