well, you can determine if it's a transmission problem even if you don't have the tools handy to tear it apart. If you remove the clutch cover and clutch bolts/springs, you should be able to turn the input shaft of the transmission relative to the output easily. The center hub of the clutch assembly is splined to the intput shaft.
You can then turn the input shaft slowly and try shifting through each gear and see if the output shaft speeds up (splines the front sprocket slide on). You should be able to go through the pattern easily while turning the input shaft slowly. If not, remove the oil drain plug and try again. I know it sounds silly...... just try it. Sometimes the oil drain plug can actually go up in and hit the shift drum.