USGS Maps-

One way to get a rough idea about this is to find a USGS (United States Geological Survey) map of your area and locate your property on that map. These are available online, as well as, possibly, your local backpacking retail outlet or similar store. These maps were mostly done decades ago so your site may have had earthwork done to it to change the topography since that time, but if you think that it has not been altered, then this may be one way to get a rough idea.

GPS Units-

Another way is with a GPS unit, which are becoming very popular these days. But again, the accuracy of these devices as far as altitude/elevation are concerned, are very limited. But they can give you, again, a very rough idea of the relative slope(s) on your property.

Google Earth-

Another method I use myself occasionally is, again, with the popular and free program Google Earth. Once you locate your property there, as you pass the cursor over different areas of your site, you can observe at the bottom left data line, that the elevation may change which you can note on the various parts of your property. Not to belabor the issue, but I must stress again that these are all methods that will only give you a very rough approximation.

Back to the "Building Envelope"-

So, now that you have assessed all of these issues that will effect your design, you have determined what will be the "building envelope" of your site. These are the limitations in size, proximity to property lines (setbacks), and maximum height, etc. of your design.

One additional issue to keep in mind too is what is called coverage. Once you have determined all the setbacks on your site, you may also be limited to a certain percentage of the lot you can have covered with building, which typically is less than the area left over on the lot after setbacks. Check with your local Planning Department to find that percentage figure for your parcel. Sometimes garages are included in coverage, sometimes not. Sometimes decks over 30" off of the ground will count towards coverage too. Also, any part of the second or third floors that hang out over the first floor (cantilevered), will usually count towards coverage as well.

In some jurisdictions, there may also be limits to what is called the Floor Area Ratio (FAR). This is the ratio of the total of all the floor space for all floors combined in relation to the size of your lot. There may also be other ratios, such as the second floor only allowed to be 80% of the area of the first floor, etc. Most of these types of regulations are intended to keep people from building one big ugly box three stories high, right out to all the setbacks of the property.

Be sure also, to check any other special "overlay" zones which may pose additional restrictions on what can and can’t be done on a particular site. It is very important to establish these "ground rules" on any project.