If you look at the heard direct list on the UI-Path32 Live webpage. This ranks the stations received in the following 3 catergories:
Green receivable under normal radio conditions
Orange receivable under moderate dx radio conditions
Red receivable under strong dx radio conditions
Note that a dx station will revert back to normal if not heard direct for more than 2 hours
I utilises a piece of software I wrote called UI-Path32 which runs on APRS. This piece of sofware monitors the packet radio frames and it records all packets which are heard direct.
This information can be used to identify changes in radio propagation. It's sensitive enough to monitor not only tropospheric enhancment/ducting but also sporadic e.
UI-Path32 also monitors DX broadcast beacons sent from amateur stations running UI-View this is also used as an indication of propagation as it shows what is the min max and average DX distance stations are getting.