An instrument and a method for identifying active channels in a communications system, especially a system operating according to the asynchronous transfer mode (ATM) protocol in which data cells are associated with virtual channels (VCs) and virtual paths (VPs). The instrument includes an ATM module with a VP/VC filter that permits capture of only those ATM cells corresponding to any selected set of VCs for a particular VP. The module also includes a processor that executes software that controls the VP/VC filter to scan through VP/VCs using a combination of slow and fast scan sequences such that channels with heavy traffic are identified quickly and channels with low bandwidth traffic are also identified, although not necessarily as quickly. To identify active VCs on a particular VP, the VP/VC filter is initially set to capture cells on all channels; as active channels are identified, those channels are disabled in the filter.