Note.:

 These wheels are only suitable for 2x2 double wheel heelys shoes only. Such as X2 , Dual up models etc..

Simple...Look inside the wheel cavity of your shoe or on the side of your wheel (also on the wheel plug) if you still have them. The size and style of wheels will be shown (for example = 

2W - S  ,  2W - M  (Other then that these wheels are not suitable) 

If still in question as to your wheel type, locate shoe model from the tongue of your shoe and visit Heelys.com's wheel chart (Original, Mega or FATS). Original wheels are nearly obsolete being from models several years back