Hi - my name is XXXXX XXXXX I'm a Criminal
Law litigation attorney. Thanks for your question.
If there is no breathalyser test, no blood test, no urine test, and no other independent evidence, then your case would boil down to what the officer testifies to. IF the officer testifies that you had red eyes, slurred speech, unsteady on your feet, that he smelled alcohol, etc., then the officer can testify that in his OPINION and based on his TRAINING that you were under the influence.
Thus, there's still a very good chance that you could be found guilty based on the testimony of the officer alone - - without any other evidence.
Also, your friend can testify on your behalf, BUT his/her testimony isn't going to carry as much weight because he/she has not had any training like the officer has to assess whether someone is under the influence or not.