How can I tell if VPN is working?

VPN does not connect you to a server, but rather another network. You can test the VPN connection simply by pinging a server on their network. Note that some VPN setups don't correctly setup the search domains for DNS for you, so you try the fully qualified name of the server that you are pinging (e.g., use 'server.company.com' instead of just 'server').

Start /Applications/Utilities/Network Utility.app

Click on the "Ping" tab.

Provide the IP address or full name of a server on the remote network.

Click the "Ping" button.

If everything is OK, you should see something like:

Ping has started…

PING server.company.com (192.168.1.1): 56 data bytes

64 bytes from 192.168.1.1: icmp_seq=0 ttl=61 time=0.396 ms

64 bytes from 192.168.1.1: icmp_seq=1 ttl=61 time=0.333 ms

64 bytes from 192.168.1.1: icmp_seq=2 ttl=61 time=0.414 ms

64 bytes from 192.168.1.1: icmp_seq=3 ttl=61 time=0.379 ms

64 bytes from 192.168.1.1: icmp_seq=4 ttl=61 time=0.278 ms

64 bytes from 192.168.1.1: icmp_seq=5 ttl=61 time=0.340 ms

64 bytes from 192.168.1.1: icmp_seq=6 ttl=61 time=0.484 ms

64 bytes from 192.168.1.1: icmp_seq=7 ttl=61 time=0.521 ms

64 bytes from 192.168.1.1: icmp_seq=8 ttl=61 time=0.345 ms

64 bytes from 192.168.1.1: icmp_seq=9 ttl=61 time=0.351 ms

--- server.company.com ping statistics ---

10 packets transmitted, 10 packets received, 0.0% packet loss

round-trip min/avg/max/stddev = 0.278/0.384/0.521/0.069 ms

If there's a problem, it will look more like:

Ping has started…

PING server.company.com (192.168.1.1): 56 data bytes

Request timeout for icmp_seq 0

Request timeout for icmp_seq 1

Request timeout for icmp_seq 2

Request timeout for icmp_seq 3

Request timeout for icmp_seq 4

Request timeout for icmp_seq 5

Request timeout for icmp_seq 6

Request timeout for icmp_seq 7

Request timeout for icmp_seq 8

Request timeout for icmp_seq 9

---server.company.com ping statistics ---

10 packets transmitted, 0 packets received, 100.0% packet loss

If you can ping their servers, then you've gotten as far as their network (I presume that you pinged a server on their network that isn't otherwise visible from the outside).

At that point, treat connectivity issues the same way you would as if you were on-site on their network and having the same trouble. You didn't explain what you are trying and how it fails, but generally speaking, issues at that point are probably related to things like authorization and authentication for whatever services you are attempting to access. If you are more specific about what you are doing and what the symptoms are, I might be able to offer some advice.

Try using the server's IP address instead of host name and see if that makes a difference. It's possible that you are not getting the DNS for the remote network (e.g., your computer can't resolve their host names). This may be a configuration issue on their end.

Depending on how the VPN is configured, mDNS (Bonjour) may or may not work over the connection. You won't see servers appear in the Finder / networks if mDNS isn't working. That doesn't mean that you can't access them, it merely means that they won't be automatically detected.

You want to use an FTP client like CyberDuck or Transmit to connect to their server. Go to server will accept FTP URLs, but you can only mount an FTP server read-only that way. Also, if you aren't getting their DNS service, you need to specify the IP address of their machine.