OK, some clarification here. One can NOT just plug an HID bulb in the socket of a halogen equiped bike and convert it to HID. It's a totally different kind of bulb and requires a ballast and transformer to operate. (I'm not even going to mention that HID bulbs do NOT work properly in housings and reflectors designed for halogen bulbs.)
In any case, just pull the old bulb out and there should be markings on it. If it's really a halogen bulb (some bozo's sell blue tinted bulbs as HID), then do NOT get a blue tinted bulb. The blue tint robs the bulb of light output and does nothing for your ability to see. It's simply a way to make a halogen bulb appear like an HID for those who are hung up on HID as a status symbol. Buy a clear halogen bulb that is high quality. A +30 or +50 will increase light output over a standard bulb. Osram, Phillips, and Narva all make these bulbs.