How can i know CDI unit is bad for honda XL185 trial bike.?

Generally you cant check them with a meter. You need to check all the other componets in that circuit and verify that they are good and you dont have any shorts or bad grounds. Or you can just go for it and try a new one but shops dont usually allow you to return CDI's. If you go to the dealership and buy a service manual for that bike it will walk you through the whole thing.