I've got an SSH server setup in a remote location, which I use to tunnel traffic from an android tablet. On the tablet, I have the Cisco AnyConnect app to connect to my company's VPN. We want to be able to detect that a proxy is being used by the tablet. Any ideas as to how I could go about doing it?
A VPN is just routing with protection. In your case, the "proxy" ensures that traffic goes first from the tablet to some server S, from which it then goes to "the Internet", e.g. to the entry point for the company's VPN. The "proxy" uses SSH but that's just between the tablet and server S; you cannot see it from the company.
What you will see, though, from the company, is the IP address of S. A relatively invasive strategy is to use OS fingerprinting on that machine (basically, scanning who is connecting) to try to guess whether S is indeed an Android system, or some other kind of OS. If you detect that "who connects" does not use Android directly, then you have shown that some intermediate system, a "proxy", is in use.
(Whether scanning machines who connect is legal is a complex subject which depends a lot on jurisdictions. Don't deploy that without taking some legal counsel.)
Now, of course, in all generality, there is no such thing as a "proxy / no proxy" dichotomy. A basic tablet user will connect over WiFi when he can, connecting to an access point which will use NAT, and that qualifies as a kind of "proxy". This already prevents the reverse-scanning explained above from really working (i.e. most of the time, the scan will see the access point, not the tablet itself).
The whole point of using a VPN like Cisco AnyConnect is to make the proxy question irrelevant: the VPN uses cryptographic protection exactly so that the paths used by data packets have no impact whatsoever on security. You should not have to ask yourself whether some SSH or NAT or whatever is in place. Thus, your question is kind of weird.