If the cameras are ONVIF compliant you could check them out with the very good Windows open-source tool 'ONVIF Device Manager' from sourceforge.net
NVRs should be able to pull an RTSP stream for recording only - you can see the RTSP string in the 'Live video' page at the bottom. This works OK with Hikvision NVRs.
NVRs should be able to connect to an ONVIF camera, for at least recording only. Some cameras (eg Hikvision) implement the ONVIF 'push/pull event notifications' that could be used to trigger motion recording. Any events generated if you activate motion detection on the camera and walk test it will show on the 'Events' page. The ONVIF version shows in the 'Identification' page.