How to locate elements in mobile app ? (findElement By class, id)

i am learning automation testing with mobile applications. When i had gone through some tutorials i found that we can locate elements using UIAutomator Viewer-- by taking screenshot and finding elements. But by taking screenshot i could not find elements of some elements. So, do we have any other ways to find elements or any other tools like UIAutomator?
Please help. If u have any good links for mobile automation testing please do forward. Thanks.