Hi,
Though it's possible to over do it on calcium, it's not a concern for most people. Only about 40% of dietary calcium is absorbed (on average). Still, it helps to know what you are taking/eating.
It is best to look for the words "elemental calcium" on labels. You usually won't see this on anything other than supplements, but you never know.
Elemental calcium refers to the absorbable component of the mineral--the part that is used by the body.
If you're a dairy eater, 3-4 servings/day will put you where you need to be. If not, look for non-dairy fortified alternatives (soy/rice milk,bread etc.)