After answering Jacks questions, take the bike out for a ride, get it up to normal operating temps then find a steep hill if possible or get the bike rolling normally then select a gear that is to high for the speed you are going, for instance say 4th gear at about 30- 35 mph, then BRISKLY roll on the throttle. If the motor bogs down and excellerates slugishly the clutch is most likely adjusted properly. If the bike doesnt excellerate but the rpms increase in response to the throttle then the clutch is slipping and will need either adjustment or replacement.
The one thing you want to be carefull of is not using the cable adjustment to try and compensate for a slipping clutch. That adjustment is soley for the preload on the throw out bearing which is roughly the size and thickness of a nickel. If this is set too tight you WILL destroy the throw out bearing in short order.