A good way for beginners (or those who knew and forgot) to check their tensions is to look at the back of the embroidery. About 1/3 of the thread - the center third -should be bobbin. If the bobbin thread spreads to the edge, then either your top tension is too tight, or your bobbin tension is too loose. Conversely, if not very much bobbin shows, your top tension is too loose or your bobbin tension is too tight.

If you have more than one needle per head, here is how you can determine which tension to adjust. Choose a letter, for instance an upper case "I". Then program as many I's in the machine as you have needles, with a colour change between each one. After you've finished the sewout, take the embroidery off the machine and look at the back of it. If the majority of them have either too much or too little bobbin, the bobbin tension is the first thing to adjust. After this is done, do another sewout and check again.
 
Tensions can be tricky because they change so much. Just switching from black to white thread can make a difference. When adjusting tensions, be more generous with your top tension adjustment than if you were adjusting a home sewing machine. Remember, generally speaking polyester threads require more tension than rayon threads.