What IP addresses does an application use

I do IT support at a large organisation and one of out network engineers asked me to go check what IP addresses are in use for one of our clients.

We have some users that work only for this client, but we have multiple /24 ranges that needs to be routed through a S2S VPN just for one java-application and one webbased tool. He has send me 3 ip addresses that he can see are being used, but wants me to check if there are more.

I don't really know how where to start, should I just install wireshark and open the tools? Or should I go and try to filter in our gateway for IP addresses that are within the /24 ranges that go to the S2S VPN.

We use Windows computers, Cisco switches and Palo Alto routers.