Three things to do/look for:
#1. Measure it. As gross as this sounds, try to 'scoop' it up and measure it. When the spit up comes out, it looks like a lot but in reality it really isn't that much. Take a look at the actual volume and if it seems like a lot (several teaspoons) there might be an issue.
#2. How is your baby's weight gain? Is he or she gaining weight, is the pediatrician satisfied with the weight gain, and is the baby following his or her growth curve? If so, then no need to worry.
#3. How does the baby react to the spit up? You have a problem if there is arching of the back, fussiness or crying, or if the baby appears to be in pain. If your baby is like mine and the spit up doesn't bother him, or he plays in it, then it is okay.
Go to www.wedmd.com and search 'happy spitter'. Our son was a happy spitter...for 9 months. It was freaking nasty. He wore a bib all the time, went through several bibs a day, we kept burp cloths handy, and I learned to wear only white or off-white shirts. Remember that in many babies the digestive system is immature and the valve covering the tube leading to the stomach is often weak. The valve flaps and flops and allows the contents to come back up sometimes.
So many people told me "oh your poor baby, he has reflux, get him on drugs, change your diet" blah blah blah, and I ignored them because he had all the classic signs of a happy spitter. As soon as my baby started eating more solid foods and spending more time sitting up and less time on his belly, it stopped.