How to detect only new and updated records (e.g. service order allocations)?

I’d like to integrate our system for scheduling of service order with NAV 2009.

I’ve enabled the Page Web Service and retrieving all service order allocations works fine.

Page: Resource Allocations (6005)
Table: Service Order Allocation (5950)

It’s a real time planning system so after reading all allocations at startup I’d like a loop detecting only new and updated service orders allocations in NAV.

Is the table trigger OnModified the way to go? How do I use it with a Web Service?

Thanks for any hint!

Best regards