best way ive found to find them by eye is to look over all the lines and where they connect to each other. being it leaked out over night there will be a greasy/oily mess at the leak point.
if you have extra r- what every. clean the pipe off good should you find a messy area, then re fill and see if its dirty the next day.
the compressor can also leak so look at that close too.
they do sell a dye kit for the a/c it will leave a red stain from the leak area. its a recharge kit with leak detector in it already.
