Oaks, which all fall under the genus (Quercus) are wide-spread trees found in temperate climates the world over. There are over 600 known species of Oak, with 55 found in the continental United States. Because there is such a wide variety of oak trees in the world, it can be hard to find characteristics that unite them all. However, they do exist:   Acorns are the easiest way to spot an oak tree. If the tree produces acorns, it's an oak.  Lobed leaves are leaves that have rounded or pointed knobs extending out from the center line. While a few oaks do not have lobes, all of the leaves are generally symmetrical around a clear median line.  Small, scaly bark. Bark on oak trees is variable, but it is generally made up of small, hard, and scaly bits of bark. This is unlike the large, flaky chunks of pines or the wallpaper-like bark on birch trees, and is much more cracked and grooved. Lobes are the bits of leaf that extend out to each side from the center of the leaf, like points on a star. White oaks have rounded lobes while red oaks have pointed ones. This is an important distinction that will cut the number of potential trees you're trying to identify in half. On red oaks, the vein of the leave extends all the way out to the edge, creating this point. Each area has its own assortment of oak species, which is often drastically different from other areas. The species of oak you will encounter changes great depending on where you are in the world, as it is rare for an oak found on the east coast to be found in the west, a southern oak in the north, etc. You can generally figure out your region with a few criteria (examples here are for Continental US):  General location -- Northeast, Southeast, Midwest, Northwest, Southwest Inland or coastal. Mountainous or flat terrain. The lobes are the sections that extend out from the center of the leaf's stem on both sides. If possible, compare multiple leaves to find the average number of lobes. A few species such as the willow oak have no lobes at all, but most oaks have multiple lobes. Count at least 4-5 leaves when identifying, as this will help when you turn to the field guide. Look at the area between the lobes and determine whether the indentations are deep or shallow. White oak leaves often have variable indentations that alternate randomly between shallow and deep, where red oaks can have dramatic, sharp indentations or none at all. The leaves of evergreen oaks are shiny and dark green all year, but most oaks will turn colors in autumn. Some oaks, such as scarlet oak (Quercus coccinea), sport brilliant colors in the fall. White oaks and chestnut oaks are often a dull brown when color comes around. If it's summer, look at whether the leaves are dark green or light green, and whether they're shiny or not, to help determine the species. Evergreen oaks and a couple red oaks such as scrub oak have smaller leaves while most red oaks and nearly all deciduous white oaks have much larger leaves (at least 4 in.). This is one of the more important distinguishing features between similar species of oak. Using the data you've collected, identify your oak tree using a tree manual or field guide. There are dozens and dozens of oak trees out there, and you can't he expected to know them all by heart. Use the above criteria to narrow down your choices, then use the guide to find what oak you're looking at. You can look through the collection of common oak trees below, or consult a Field Guide, found through the US Forestry Service.  Turn to the appropriate section. Most guides are divided into a red oak section and a white oak section Narrow your selection down to the oaks specific to your area. A good guide should have distribution maps for each species. Once you have a list of possibilities, look at the pictures for each one to determine your tree.

Summary:
Differentiate oak trees from other species. Look at the tips of the lobes to determine if you have a red or white oak. Consider your geographical region. Count the lobes on each leaf. Measure the indentations between the leaves. Look for color changes in the fall. Measure the overall size of the leaves. Identify any unknown oaks using a Field Guide by the US Forestry Service.