It's important to have an approximate knowledge of the depth of the local water table when you are planning to dig or drill a well. Knowing how deep you will need to dig can give you an idea in advance of the project's cost, and whether it is cost-effective in that location. It's also important to be aware of groundwater levels when planning excavation, because digging in the wrong location could lead to future problems, such as moisture issues in a new basement.
Examine other wells
Ask your nearest neighbours if you can look at their wells. Established dug wells can be examined visually, while drilled wells, which are often much deeper, may require more complicated methods such as an electric depth gauge.
Write down the depth at which the water is found in each of the neighbouring wells. Using a topographical map, adjust for the elevation of the surface of the land at the location of each of the wells, and come up with a standard measurement above sea level for the water table at the location of each well.
Extrapolate, using this information to get a reasonable estimate, where the water table lies at the location of your well.
Use a dowser
Hire a dowser to test your land. Dousing is a process in which a trained or "gifted" person goes over the land with a forked stick or a pendulum and, through techniques that are not scientifically quantifiable, determines the location of underground water. Dousing is a controversial activity, with some people swearing by it and others dismissing it as nonsense. Use your judgment.
Ask the dowser to go over your land and determine the best location for finding water at the closest distance to the surface, to minimise the expense of drilling or digging. Ask the dowser to examine your entire property and to identify multiple possible locations for a well, in case your first choices are not usable due to location, siting of the house, or other reasons.
Ask the dowser to estimate not only the location of the water, but also its quality and how many gallons per minute the well might produce. If you are dubious about the dowser's conclusions, you can attempt to confirm or discount them by subsequently drilling test holes or using other more scientific methods such as an examination of topography and local water tables.