We've been pulling together a database of mods, and are currently planning on using transmission to identify motorcycles. I don't think there is anything in insim that can identify a motorcycle, either have to get the data somehow, or use their api
I haven't checked the values, but the IS_NPL packet gives you the tyre compounds for "all 4" wheels - maybe 2 of those values are equal to TYRE_NUM, which would indicate a motorcycle?
Sadly I did just check, and I'm getting the same value for all tyres, even non-existing ones (e.g. 0 (R1) for "all 4" tyres of the Reaper X1 bike, or 5 (road normal) for the 3-wheeler Hadraplan). At least that's what I get with the default setups.