Well, first you could check with your router. It may have a page showing upnp status.
upnpd on linux adds entries to filter.FORWARD and nat.PREROUTING, plus it logs to syslog (mine goes to /var/log/debug).
Another option is to set maxconnections to a number higher than 8. The client usually tries to maintain at least 8 peers by making new outbound connections. If you set maxconnections higher, it will accept incoming connections until it reaches that limit. So, if you see 10 peers in your client, you know that at least 2 of them are incoming, and thus, upnp is working.
Topic: How to Tell if Miniupnp is working (Read 1003 times)
I've compiled the libs, and set the make switch to on . . . but the compiled binary is not much larger and the program doesn't seem to have better connectivity. How do I tell if uPnP is active? And what to do if not?
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