How do I know if my baby is spitting up too much?

Very large spitting. If the spitting color is brown, bright red or green, you should talk to your pediatrician immediately as further evalaution needs to be done. Also, if spitting is large and shoots out of baby mouth or baby is not gaining weight, see your pediatrician.
It is normal. Most babies have some degree of "reflux" and stomuch contents come upto baby's mouth and baby spits up. It is very commom to have spitting and generally it will get better as baby is growing. As long as baby is keeping most of the feedings down and gaining weight, don't worry about it. Talk to your pediatrician during your baby's next visit if your are still worried about it.
Baby "spit up" All babies have a certain ammount of "spit up" that resolves over the first 6 months of life. As long as a baby gains weight, is happy and developing normally it should not be a problem. Make sure you do not over feed your baby, and that you do not rock him or put him in a swing after feeds. Keep your baby in his infant seat for about 30 minutes after feeds, it will keep him at an elevated angle.
More than once a day. Many babies don't spit up except a few spoonfuls with a burp, or after a big feeding; and that may only happen once every day or two. So, if a baby spits after most feedings, or spits more than a few spoonfuls, a doctor should evaluate the symptoms. Also, if a baby seems fussy or uncomfortable when eating or being spitty, the parent should talk to the doctor.
Very common. Babies spit up because the valve that keeps food down has not fully matured. This valve, the lower esophageal sphincter, connects the esophagus to the stomach and is responsible for keeping food in the stomach.
If she is not... ...Gaining enough weight. It's not how much comes out that matters; it's what stays in.
Weigh her. All babies spit up. Most of the time the amount is small and leads to more laundry but is not a medical problem. Some babies seem to spit up larger volumes. It is usually due to over-filling the stomach and is a reaction to that bigger volume of milk. If your baby acts normal, is happy and playful, has a good appetite and is gaining weight, then it's not too much.