Get the Model Identifier and some specs (GHz, RAM, HDD, GPU) from your neighbour's iMac and look up more specs here, then go to Mac2Sell or eBay and look for that model via specs (example: 24" iMac, 2.8GHz, 2GB RAM 500GB HDD) to get a bearing, as there are many 24" iMac models to get a bearing.