How one can say that GPS Position is Good/Bad during rum-time without visualizing on Map ?

Hello Everyone ,

I am working on development of Automotive Dead Reckoning (ADR)

I am using low cost GPS Receiver + Vehicle Motion Data ---> as inputs to the ADR System. The System has got the Vehicle Model , Adaptive Filters which does estimates the Lat-Longs based on previous state.

The Flag which drives the ADR System Filters , we call it as a confidence flag ... The confidence flag is a function of {No of Satellites Used , Positional Accuracy , Running Standard Deviation , HDOP , VDOP ,CEP} --all from GPS NMEA Data and the Distance between the Vehicle Dynamics Prediction outputs and GPS outputs..
This confidence flag helps me in switching between GPS Lat-Longs and Predicted Lat-Longs.

Does anyone suggest any other parameters through which I can know if the Lat-Longs coming via GPS Receiver are bad ???

To the point , How can anyone say if the GPS Positions outputted by the receiver are good /bad ?? What can be the parameters in Run-time that Once can say its a BAD GPS Postions and I cannot Trust it anymore.
As far as I know , if its going through a tunnel or in a bad weather or in Downtowns (urban cnayons) , it loses accuracy and starts giving wrong positions
I am open for a discussion on personal email too .
let me know if you have questions.