You can actually tell from the licence number, at least for licences issued in the last decade or so.
A UK FCL licence valid for use on part 21 aeroplane has the licence number starting UK.FCL.PP, UK.FCL.CP or UK.FCL.AT
A UK national licence issued since the end of JAA at least, has the licence number starting GBR.PP, GBR.CP or GBR.AT.
There is presently in ORS4 1471 (
link ) a temporary exemption allowing national licence holders to fly G-reg part 21 aeroplanes for a limited number of hours, and there's every hope that in some form that will be extended. However it prevents any instrument privileges on that licence being exercised on part 21 aeroplanes. Which has no logical basis whatsoever, but seems to keep the DfT happy on some procedural grounds or other.
G