Get data updates in VB application only when it is a real update, not refresh

I found a similiar topic on the forum but a bit different...

We are using pooling to retrieve data from opc iFix, so even there is no updates, we still get updates, and we intend to keep it in this way.

But how do I retrieve real time data from pi server to my visual basic application only when it get updated(not refreshed) or maybe there is a flag in PI telling us which data is a update and which data is only from refresh?

Appreciate all help!