When there are components from VRCSDK2 and VRCSDK3 in the same scene, the builder tells you there are conflicting objects.

"The scene contains components from the VRChat SDK version 2 and version 3. Version two components will have to be replaced with their version 3 counterparts to build with SDK3 and UDON."

However, the "Select" button does not display all the objects that are problematic.

It shows it in Unity's multi-object selection UI, but there is no way to work out which objects are conflicting.

Note for devs: Rather than select the component, maybe select the GameObject(s) that the components are in.