Personal contact: As you approached the vehicle, what did you see? As you talked to the driver, what did you hear, see, and smell? How did the driver respond to your questions?
Pre-arrest screening: How did the driver do while performing the field sobriety testing? What did he or she do wrong during those tests? Is the impairment caused by alcohol, drugs, or illness? Do you have enough probable cause for an arrest? If not, what other options do you have per your agency policy?
2. Ask questions. Ask, How much have you had to drink? Where are you coming from? Where are you heading? The more questions you ask, the more reasonable suspicion you build and tip over into probable cause. This becomes even more important when there is a pass-off from one officer to another; especially if you have your own DUI unit. The more the initial stopping officer can get before making the call to another more trained officer, the more it will help later on in court. "I thought he was drunk so I called Officer Smith," is never going to be enough. When you are talking DUIs you need to be talking in details.
3. Be observant. You need to look for signs of impairment through your entire stop. The obvious one is looking for empty cans or bottles of open alcohol. Is the driver fumbling around with her wallet when you ask her for her drivers' license and registration? Did he pass over his license several times before finally handing it over to you? One that is often overlooked is evidence of "the club hand" (bracelet or stamp sometimes used to mark club entrance). It's important because it may not be there the second time around when the person realizes it's a bad thing to still have on and he or she rips it off. How about checking if the engine is still hot? Is the key in the ignition and left in the "on" position? You need to be curious and have a sense of urgency about it. You need to find evidence of physical control.
4. Wheel witness. If the call is a traffic crash, try to hang on to the wheel witness if at all possible. If the person has to leave, try to get a written statement before. At a minimum, get a name, address, phone number, and e-mail address. You can't charge DUI if you can't prove the suspect was in control of the vehicle (and the vehicle was operable) at the time of the alleged offense.