A method is described to allow objective evaluation of intra-operative gamma probe performance for the task of sentinel lymph node localisation. The method uses simple simulation based upon standard sensitivity and spatial resolution measurements at depth in water, with technetium-99m sources. The aim is to predict the minimum separation between the injection site and lymph node required to allow the sentinel lymph node to be identified in the presence of high injection site activity. The simulation methodology allows rapid investigation of probe performance for a range of node and injection site activities, and a range of node and injection site depths, without the need to perform a large number of physical measurements. Examples of practical performance simulations are given from five probes, showing that nodes at less than 115 mm from the injection site may be poorly localised, with even the best performing probe requiring at least 51 mm separation to allow detection in the high background from the injection site. This method provides data to allow the ranking of probe system performance in terms of the practical task of sentinel lymph node localisation, rather than arbitrary ranking based upon basic physical performance measures such as spatial resolution and sensitivity. The best probes allow sentinel lymph node localisation at between 20 and 30 mm closer to the injection site than the poorest performing probes, for situations which represent intra-operative localisation in melanoma and breast surgery. The method is also shown to assist in optimising system settings such as energy detection thresholds, and may allow users to understand the limitations and capabilities of intra-operative gamma probes.