How to tell if packet is VoIP?

Depends on the protocol the VoIP stream is based on.
If the VoIP is in ordinary RTP and if your sniffer is able to automatically decode RTP/RTCP (like Packetyzer V2.0 or higher), you’ll see immediately that there’s VoIP traffic (by looking at the codec of the RTP-Payload).

If the VoIP stream comes from a Skype phone, you’ll see nothing specific in the traffic except that there’s a huge load of packets per second exchanged between two certain peers (i.e. there’s any kind of a stream).

Some IP-Phones use other protocols than RTP to carry VoIP payload, e.g. the IAX2 protocol “mini packet” (which only carries a 4 byte header before the payload). If your sniffer is able to decode that (like e.g. Packetyzer), again you will see immediately.

As a conclusion one could say that it all depends on the way the VoIP traffic is carried over the network, and that usually depends on the technology used with the specific IP phone.

Regards

Frank