I'm using ARCore to build my android app, where I allowing users to place anchors. I need to be able to check if the Anchor is in the current frame. Any idea how can I do it? Thanks!
If you're using ARCore, you're probably doing frustum culling where you don't render objects that aren't within the viewable space, an optimization used to stop you from making gl calls to render "unviewable" elements of your scene.
If you have access to the objects after the renderer calculates this, then you can use that value.
Another way you can do this, is by grabbing the Camera and getting the View and Projection matrices. Then you can project the anchor coordinates onto 2D screen coordinates and if the calculated coordinates are outside the screen (ie. x/y values are > or < the screen width/height). You'll have to account for objects that are behind the camera too (the dot product between the camera forward and vector from camera to anchor should be positive).
https://developers.google.com/ar/reference/java/com/google/ar/core/Camera.html.