How do I backtrace a landscape catch basin with only the exit point?
You can find the end, or at least the first leakage point in the line, with one of these:



It's intended for cleaning drain/sewer pipe. How it works is the small bladder/balloon will expand under the water pressure of the hose (garden hose water source) and has a small exit point on the other side of the balloon. So it will expand to the pipe size then fill it up. This way will use a decent amount of water but on a dry yard day should work fairly well.

Item/Picture source but they come is all different sizes and should be available at your local hardware store. I would measure the pipe first and buy the largest/closest size one you can, this is a 3" to 4" pictured but it may not seal a 4" pipe well until it has a certain amount of back pressure.



Same concept different approach, you can get 4" (or 3" depending on what you have) pipe fittings and step it down to a garden hose fitting.