There are several different types of field sobriety tests that law enforcement officers use during DWI/DUI arrests and stops.
However, only three of them are recognized as reliable as standardized by the National Highway Traffic Safety Administration.
Typically, this consists of the officer holding up a pen, and telling you to follow it with your eyes. The officer will be looking to see whether or not you can follow the moving object smoothly, whether your eyes involuntarily move while focused in the extreme peripheral, and how soon they involuntarily move. - The Walk-and-Turn Test
This is called a "divided attention test," meaning you must listen to instructions while performing a physical task. The standardized version of this test requires you to take nine heel-to-toe steps forward, pivot, and then do the same back. You will be asked to count the number of steps you've taken out loud. The officer will look to see if you miss steps, have difficulty balancing or turning, etc. If you use your arms to balance or fail to complete the test, the officer will count this as failure. - The One Leg Stand
The officer will instruct you to raise one foot six inches off the ground, count to 30 out loud, and then put your foot back down. They are required to ensure that the test is given on a flat, hard and dry surface. If the you are over 65, have a physical impairment, or are more than 50 pounds overweight, this test should not be used. In most cases, these tests are designed to be standardized, however you can see how subjective the officer's interpretation of your performance can be. Other (non-standardized) tests you may encounter include reciting the alphabet (or just a portion), counting the number of fingers on an officer's hand, counting backwards, or closing your eyes and touching your finger to your nose. Will these non-standardized tests hold up in court? It's hard to say. Your lawyer may be able to convince a judge to throw out the evidence from these tests, as they are so often unreliable.