I just placed pvc pipe with holes in the garden for drainage. How do I know if it will work?
There is runoff water that comes down to my backyard when my neighbor waters her front yard. The area that I inserted a drain pipe is wet and muddy so I tried my best to construct a french drain. I inserted a sleeve on the pipe and placed the holes up. I haven’t seen any water drain out of the pipe yet and is concerned if I need to adjust for slope.

I think the holes need to be on the side. Did you line the trench with rock as well as layering in landscape matting to prevent dirt from filtering in and plugging the holes? Is the pipe properly sloped to drain, i.e about 1 inch per 36 inches should work well.
To work properly, the perforated PVC, should be placed within a filter sock or wrapped in filter fabric. It should then be placed within a gravel pocket that is also encased within a filter fabric sock or cloth. This will keep all the dirt, garbage, and misc. material away from the filtration system. Be sure you have at least 1% slope drainage at minimum on that pipe also to drain wherever your taking the water. The gravel pocket also works as a seive to bring the excess water to your drainage system.