Outline your plot. You should have done this prior to beginning your script. If you haven't, do it now.
Pinpoint your A and B plots.
Find the location within your A and B story lines after the exposition and conflict have been introduced, but before there is any real resolution.
Check if this point is around the middle of your script. If not, it means your script is probably off balance.
Rearrange your script so that there is enough action, exposition and conflict in the first act to make the reader of your script want to continue to see the resolution. However, ensure that the second act doesn't cut the tension off too soon but rather draws it out until the end of the script.
Read over your script. When you get to the act break, do you want to keep reading? Are you asking yourself, "How will this character get out of this situation?" If so, you have successfully written an act break.