I have a few of these excellent little units
This 2002 review shows them to have been eye wateringly pricey at over 300 quid (under 20 quid on Ebay now) and it says it does WAAS.
I use one of these in the plane and have done since 2002 because it is unique in having connections for both power and an external antenna, which is exactly what you want for a velcroed bluetooth GPS fed from a rooftop antenna.
However there is no obvious way to tell whether it is picking up WAAS/EGNOS because I have none of the ancient apps for it. All I have is basic apps which show the constellation (e.g. Oziexplorer). How would one tell?
Along with nearly all consumer GPSs, then and since, it has the ~160ft altitude error.
Most standard applications can check it for you; or you can just look at the $GPGSA and $GPGSV sentences. GSV lists the satellites your receiver is capable of receiving at the moment, and GSA lists those actually used for the fix. EGNOS satellites are 33, 36, 37, 39, 44 and 49 (some of them have been retired – the data from different sources are a bit contradictory – but these numbers won’t be reassigned to other services anyway).
data from different sources are a bit contradictory
They certainly are. Reading here there is currently 3 sats numbered 33, 37 and 39, with only 33 and 37 used for business at this time (whenever that is, the text is undated). However, my old handheld is now receiving number 36 and no others.