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I'm so sorry. Generally you can tell if they've passed once you can't feel a heartbeat anymore. Sometimes animals do seem to go into a stage of 'shutdown' in their last minutes where they are completely unresponsive as well. ((HUGS)) to you, and so sorry for your loss. Seven is a great age for a guinea pig. Unfortunately small animals often go from seemingly okay to passing away without giving us much warning or time to prepare.