Unlatched fire doors are obviously solely reliant on the self-closing device to provide an opening resistance which in turn, maintains the leaf securely against the frame stops in a fire situation.  However, I cannot find any reference as to what this minimum force should be.  Is there a guideline minimum opening resistance and how should this be measured? 

In more extreme cases, it's generally easy to spot such defects (e.g. communal fire doors opened by draughts) but there must surely be a more scientific method to identify potentially dangerous unlatched fire doors which may not exhibit such obvious failure symptoms?

Thanks in advance. 

The standard for door self-closing devices is BS EN 1154.  For a fire door the device should be minimum power-size 3 and have controls for adjustment of closing speed and damping action.  Section three of the document 'Hardware for Fire & Escape Doors - Code of Practice' at  http://www.firecode.org.uk/Code_of_Practice_hardware_for_fire_and_escape_doors.pdf#:~:text=Code of Practice%3A Hardware for Fire and Escape,on fire-resisting doors and doorsets%2C and escape doors.    provides detailed information.