The emitter flow and spacing are usually printed along the tubing or on the spool's label, for example "0.5 GPH, 6 in spacing" (GPH is gallons per hour). The listing where you bought it will have them too. Add up all the line laid in the area.
How the numbers are worked out
A drip line's emitters sit at a fixed spacing, so its length tells you how many there are. 40 feet with an emitter every 6 inches has 80 emitters, and at half a gallon an hour each, the line puts out 40 gallons an hour.
Drippers and soaker hose work the same way: the count times each dripper's rating, or the length times the rating per foot. Sprinkler heads are rated in gallons a minute, so a head's rating times 60 is its gallons an hour.
One inch of water over one square foot is 0.62 gallons. So 40 gallons an hour over a 4 by 8 foot area (32 square feet) comes to about 2 inches of water an hour, measured the same way as rain.
Ratings assume a set water pressure, so a line can run higher or lower than its rating. Drip also wets the soil along each line more than the gaps between lines, so the inches are an average over the area.