It depends on where you are and the law there. As people have mentioned, generally first time offense can avoid jail time on a conviction and get probation and an education program along with fines.
Granted there's no blood test to prove you were under the influence marijuana, the state can try to prove their case using other evidence such as erratic driving, odor of marijuana, failed field sobriety tests, etc. You should talk to your lawyer regarding your options and if you don't have one, you should seriously consider obtaining one.
If you are going to trial on this charge, to the other members, the charge was already proven and accepted by the prosecutor's office. Although I never heard of going to "trial" for being under the influence from smoking pot, you usually only have an appearance before the judge with your attorney in a room filled with other people charge with one thing or another. Depending if this is your first offense, your charge could be adjudicated and you could be given community service, ordered to pay all fees and court costs and could also have your license suspended for a short time. Usually there is only a "trial" when a felony drug offense was committed.