How do I check the rectifier to see if its working properly/

I will assume that you have a multimeter and are familiar with its operation.
You can check the diodes in the rectifier by checking from each of the three stator terminals to the positive then the negative terminals.
I'll call the stator inputs to the rectifier 1,2,and3. I'll call the positive and negative terminals P and N.
Using the diode check on your meter, first check is with the positive meter lead on P and negative lead on 1. The meter will either read open or continuity. At this point it doesn't matter which. Now swap the meter leads, positive lead on 1 and negative lead on P. The reading should be the opposite of your previous reading.
Now repeat with 2 and 3. They should have the same readings as 1.
Now repeat the same tests with the positive meter lead on N. The results should be the inverse of the first set.
In all you will be making 12 checks.
Here is an example:
Meter Leads Result
Pos Neg
P 1 Continuity
P 2 Continuity
P 3 Continuity
1 P No Continuity
2 P No Continuity
3 P No Continuity
N 1 No Continuity
N 2 No Continuity
N 3 No Continuity
1 N Continuity
2 N Continuity
3 N Continuity
Remember your results may be reversed.
If you have not already done so, you should check your stator for sufficient AC output. When revving the engine, the lead to lead output from the stator should be in the neighborhood of 60-90 ACV.
If your stator is good and your rectifier checks out, but you are still not getting charging voltage(13-15 VDC) at the battery, the voltage regulator portion of the regulator/rectifier is bad and the unit will need to be replaced.
Hope this helps.
James