Water temp

[Brandon Rogers]

Hey guys-
I've posted a couple times regarding water temps, but I still have questions/concerns...Here's the deal:  Motor is a 3B with a VDO 250F (120C) max water temp gauge, signal comes from MFTS (rated at 118C per ETKA).  The engine seems to run too cold _assuming_ the gauge is giving a correct reading.  It reads about 170F (77C) in most conditions.  This summer it would get to about 180-185F (85C) in hot hot traffic.  I put a factory "blocker" behind the grill, in front of the Aux Rad with little effect.  

What is the best way to test if the gauge and/or sender is giving an accurate reading?

Thanks-

Brandon
'84 urq 20V