You don't even need to worry about what someone can physically provide. All bids by market participants can (arguably should) be checked for extreme outliers. A

You don't even need to worry about what someone can physically provide. All bids by market participants can (arguably should) be checked for extreme outliers. A bid of multiple GW from a participant that usually only trades hundreds of MW should immediately set off alarms. It only takes a phone call like "hey, are you sure about this?"