You can find out when the County Attorney charges you.  The charge generally includes the amount allegedly in your possession.  The amount typically determines whether the crime is a misdemeanor or felony.

For example, in Arizona, possession of less than two pounds of marijuana is a class 6 felony. The Office of the County Attorney may charge a class 6 felony as a misdemeanor at their discretion. See the LawForKids.org Youth Laws Section.

Usually, you must be in possession of a useable amount. In Arizona, there is not necessarily a set amount that is considered a 'usable' quantity of marijuana. The state has the burden to produce evidence that the marijuana is in a form and amount that can be used under known practices. What this means is that the prosecutor usually will have an expert, often a law enforcement officer, testify as to how people commonly use marijuana and how much of it is needed.

In one case, 0.3 grams of marijuana (a very small amount) was considered to be a usable amount. (State v. Murphy (1977) 117 Ariz. 57, 570 P.2d 1070).