The Surfing/Flying Pikachu can be caught using the pokewalker and travelling along the yellow forest route.
When using the pokeradar, you are garenteed to catch a flying/surfing pikachu if you have walked 10000steps+ AND when searching in the grass, catch a Pikachu that appears when "!!!" appears above the grass.
If you catch a pikachu that appears in a patch of grass with one "!" it is impossible to be a surfing pikachu. If you catch one with two "!!", then there is only a very slim chance you will get one.
So my suggestion to you is to wait until you see three "!!!" before you attempt to catch.