Dip a cup into the puddle and scoop up as much water as you can. Pour the water into a large measuring cup. If you run out of space in one measuring cup, add water to a second cup and so on.
Continue scooping up the water until the puddle has been completely emptied of all water. When you get down to the last remaining bits, use a syringe or similar device to help you extract the rest of the water.
Examine the measuring cups to determine the volume of water. If you have collected more than one measuring cup, add all the measurements together to get your volume. Most measuring cups will depict the volumes in either milliliters or in cups --- both are acceptable measurements of volume.