COLD running 617??

617 turbodiesel.

Previous owner is a family member and swears the T-stat is "not old." Last few weeks the car has seemingly never come up to operating temp. This morning was quite cold (18F) and on my 40-mile drive, temperature gauge never got over 60C.

Heat is marginal in the car but I know the monovalve is bad and I plan to fix that issue this week.

Are there other ways besides the presence (or lack thereof) of cabin heat to tell if I'm looking at a cold-running 617 or just an inaccurate temp gauge? Or would you recommend just replacing the T-stat?