The police must have probable cause to arrest someone for DWI.  Typically, they obtain probable cause by having you perform field sobriety tests.  Usually, these are done before any request for breath or blood samples are made.  The NHTSA (National Highway Traffic SafetyAdministration) provides guidance for police officers who use field sobriety tests.The 3 tests listed below are the standardized tests that most departments use to determine whether an individual is intoxicated:

• HGN: Horizontal Gaze Nystagmus Test.  This test is done to see if there is any jerkiness in your eye or lack of smooth pursuit.  They hold an object (a pen, or something similar), in front of the subject,and ask them to hold their head still and follow the pen with only the eyes.  In order to get a valid result, the test must be performed correctly.  Often the tests are improperly administered.

• Walk and Turn (WAT).  This test is where the officer has you walk an imaginary straight line, turn a certain way, and then walk back across this imaginary straight line.  If, in the officer's opinion, you step off this invisible line, this is one of the clue's they count against you.  They also test your ability to follow directions, your turn, your balance,swaying, among other things.
  

• One Leg Stand (OLS). This test has the subject standing on one leg for 30 seconds.  The officer judges the person's ability to follow directions, as well as their ability to actually perform the test.

Generally, the penalties for DWI include:

• 1st offense: a first-offense conviction includes a fine not to exceed $2,000.00 and/or the possibility of serving jail time from 3 days to 180 days, and a driver's license suspension of 90 to 365 days. (Class B Misdemeanor).
• 2nd offense: the maximum fine increases to no more than $4,000.00 and/or jail from 30 days to one year, and a possible driver's license suspension ranging from 180 days to 2 years. (Class A Misdemeanor).
• 3rd offense: here, you may receive a fine up to $10,000.00 and/or 2 to 10 years of imprisonment, and suspension of your driver's license ranging from 180 days up to 2 years. (3rd Degree Felony).
• DWI with an open alcohol container (first offense): In addition to the penalty referenced above you face a minimum 6 days in jail and a fine of no more than $2,000.00. (Class B Misdemeanor).
• DWI with an accident where serious bodily injury occurred as a proximate cause of the intoxication: this crime is called intoxication assault, and upon conviction you may serve a minimum of 2 years up to a maximum of 10 years in jail. Additionally, you may be fined up to $10,000.00. (3rd Degree Felony).
• DWI where a death has occurred as a proximate cause of the intoxication: here, the crime is intoxication manslaughter. Upon conviction you might have to pay a maximum fine of $10,000.00 and/or be imprisoned from 2 to 20 years (Intoxicated Manslaughter or Manslaughter with use of Deadly Weapon are both 2nd Degree Felonies).
• In some of the above minor classifications you may be eligible for probation, but there is no guarantee that you will receive a probated jail sentence or fine.

Generally, the penalties for DUI as a minor, in addition to suspension of your driving license, include:

• 1st Offense: Class C misdemeanor. If you are under age 18 the court will require your parent or guardian to be present with you at every court appearance. Furthermore, the parent or guardian can be forced to attend by the court. Upon conviction, a minor may be fined, and will be required to complete a minimum of 20 and a maximum of 40 hours community service related to education about or prevention of misuse of alcohol. Additionally, the minor will be required to attend an alcohol awareness program sponsored by the Texas Commission on Alcohol and Drug Abuse within 90 days. Furthermore, if the minor is under age 18, the court may require the minor's parent or guardian to attend the program with the minor. If the minor fails to complete the alcohol awareness course within the 90 day period the court may impose an additional license suspension up to six months. For a first offense the minor may receive deferred adjudication, however, an order of deferred adjudication for DUI is considered a conviction. If the minor receives only one conviction for DUI (while a minor), that conviction may be expunged from his record after his 21st birthday.
• 2nd Offense: Everything for a first-offense conviction applies to a second conviction, with the following exceptions: The minor will be required to complete a minimum of 40 and a maximum of 60 hours community service related to education about or prevention of misuse of alcohol. Furthermore, a second or any subsequent conviction may not be expunged from your record, however, the minor may still receive deferred adjudication for a second offense.
• 3rd Offense: Class B misdemeanor. For a third offense the minor will still be required to complete between 40 and 60 hours of community service, however deferred adjudication is no longer an option. As with the first two offenses the minor will be required to attend an alcohol awareness program within 90 days of your conviction, and if the minor is under age 18 a parent or guardian will be required to attend both the minor's court appearances and the alcohol awareness program. If, on the other hand, the minor is 18 years of age or older at the time of his third offense, the penalty is much higher. Specifically, the minor will receive a fine ranging from $500.00 to $2,000.00; confinement in jail for up to 180 days; or both a fine and some jail time in addition to a license suspension.