[RTP] How to check if NAT router knows what ports to open?

Hi

I'm wondering how to check if a NAT router is SIP-friendly, ie. it can analyze SIP packets sent from the client to the other side?

Should I look for UPnP, if indeed that's the indication that a NAT router can open incoming ports dynamically? The reason I ask, is that I have a Netgear router connected to an ADSL modem, with UPnP activated... but I still had to map ports for SIP clients inside to carry conversations.

Thanks.