Use this tool to check the number(s) in question. If they come up as IN NETWORK, then it will count as M2M.
If you are using Mobile IM - then all messages count as out of network.
Another way, but I wouldn't really rely on this, is that the green arrow indicating a sent message will turn to a green checkmark indicating the message was received, and that only shows if the message is sent/received by a Verizon number. YOur recevied messages would not give you any way to tell.
Also, on your online accounting, the message total will be your out of network....the M2M will not show in the total (it doesn't seem to in mine anyway). When you view messaging detail it will show everything and break down in network and out of network.