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You can run an inventory report (Reports - Additional Reports - Inventory). Or, the serial number can be one of your columns when you're in the Monitor tab. If it's not there already, click on the spreadsheet looking icon towards the upper right with the green plus sign on it to modify the columns that you can see on the Monitor screen. You can add the serial number column if it's not already there.
Is it HMOL? Then you can export the ap properties included the serial numbers. You find it at configuration -> Aerohive AP's -> select the ap's (or all ap's) -> Device inventory... button -> export. Hope this helps.