How to find out user who does not exists

Hi All,

I am working on migrating a Moss 2007 list to SharePoint 2013(On Prem) list. The list has people picker fields. Many of the users in this column values do not exists in the system. Is there a way we can find out which users do not exists anymore ?

Please not I do not have access to server so please suggest any client side script or api that is available to validate users that do not exits.

Thanks,

Monali