What communication can be intercepted by network I'm connected over VPN to?

Solution 1:

That depends on how you route your internet connection. In most cases, you'll just route addresses from the client's network (typically in a subnet of 10.0.0.0/8).

Type route in a console to see your routing table. The first default entry determines how your regular internet connection flows. For example, the following configuration does route over the VPN:

0.0.0.0         172.16.0.5      0.0.0.0         tun0 # default via tun0(=VPN)
172.16.0.5      0.0.0.0         255.255.255.255 tun0
1.2.3.4         192.168.1.1     255.255.255.255 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   wlan0
0.0.0.0         192.168.1.1     0.0.0.0         wlan0 # default via wlan0(local)

A simple alternative is checking whether your IP address changes when you connect to the VPN.