Fluoride has many benefits.  It is most important to the teeth because it provides minerals that strengthen weak spots on the surfaces of teeth and prevents cavities from forming there.  In fact, fluoride acts on teeth in two ways.  First of all, when you use fluoride toothpaste or mouthwash, the fluoride passes over your teeth to protect the teeth from decay.  Secondly, when you ingest fluoride through water, food or supplements, your body incorporates the fluoride into developing teeth which makes them stronger and protects them from decay.  The best source for fluoride ingestion is in the water that we drink.
Fluoride is naturally found in minerals, rocks and soil.  It is absorbed into the groundwater as the water passes through the soil and the fluoride is dissolved into it.  The amount of fluoride in the water varies from one location to another and many water companies supplement fluoride into the water sources in order to reach optimal levels of fluoride. 
The right amount of fluoride is needed to protect teeth. Too much fluoride can be harmful to teeth and bones and too little fluoride will not protect them.  Mg/L is the measurement used to determine how many milligrams of fluoride are in each liter of water.  In order to protect teeth from decay, the best level of fluoride in drinking water is between 0.7 mg/L and 1.2 mg/L.  The amount necessary in your area will depend a lot on the climate of your area.  For example, in areas where temperatures are high, people are expected to drink more water so less fluoride will be added to the water to prevent overdose.  On the other hand, in colder climates where people tend to drink less water, more fluoride is added to the water in order to ensure that people get the amount of fluoride necessary.
If your fluoride level is above 2.0 mg/L, your children may experience a condition called dental fluorosis.  This is not a health risk, but rather a cosmetic problem only.  Children will experience stained or pitted teeth.  However, if your fluoride levels reach above 4.0 mg/L, it is not safe to drink.  Over time, drinking water at these levels can cause skeletal fluorosis which is a serious bone disorder.
In order to find out if you have fluoride in your drinking water, you must first consider the source of your water.  Do you mostly drink bottled water or tap water?  And if your main water source is from the tap, is it city water or does it come from a private well?  Bottled water providers vary greatly in the amount of fluoride they add, but you can find that information on the labels. 
If you own your own well, you can have your drinking water tested at a state certified laboratory.  You will need to collect a sample to take to them and they can let you know how much fluoride is found there.  You can do the same if you have city water provided, however, they are required to have the same tests done periodically, so you may consider just calling them up and asking for that information.
If you find that there is too much fluoride in your drinking water, you can purchase a treatment system that removes the fluoride.  If your fluoride levels are less than 0.7 mg/L then you are considered at risk for developing cavities because those levels do not protect against them.  In this case your dentist may recommend you take fluoride supplements, usually in the form of drops or tablets.  You will need to consult with your dentist about the proper levels of fluoride for your family.