Unity's Debug.DrawRay and Debug.DrawLine are valuable tools for debugging 3D (and 2D) information.
By drawing lines in the Scene and Game view you can validate assumptions about positions and directions used in code.
Lines not drawn continuously will appear for a single frame, to counteract this a duration can be provided as the fourth parameter.
Make sure that the variables used in your draw functions are the same as those used by the functionality you are debugging.
If lines don't appear, ensure Gizmos are enabled for that view
Note that DrawRay takes a position and a direction.
Scaling a normalized vector will produce a vector with that length. This can be done here to make the output more visible.
Unity, huh, how?
Debug.DrawRay(transform.position, direction * length, new Color(0.6f, 1f, 0.2f));If you're passing two positions to this function the results will be unexpected. Use DrawLine instead.
Debug.DrawLine(transform.position, Target.position);If you trying to debug physics queries (overlap, check, cast), you can use the Physics Debugger ().
Your version of Unity may not have the Queries tab, you would have to update to draw queries using built-in functionality.
You can show collision geometry in the scene by enabling its visibility from the overlay in the bottom right of the Scene view window. This may help if it's unclear what's being hit.
I've created a custom debug drawing package called Vertx.Debugging for drawing wireframe shapes.
It has drop-in replacements for physics functions (casts, overlaps, hits, checks), many extra shapes, and can draw labels in the scene.
It is designed for editor debugging and most functions will be stripped when built.
If you are using Entities (ECS), Unity Physics (com.unity.physics) has PhysicsDebugDisplaySystem, which contains static helper methods for drawing wireframe shapes from jobs.
If you do not understand what Entities is, you should not use this package for debugging.
There is also the Physics Debug Display authoring component that has toggles for similar debugging to the Physics Debugger.
Raycast Visualization is a drop-in replacement for Physics queries.
It is designed for editor debugging and most functions will be stripped when built.
ALINE is a paid alternative that is intended to draw shapes in the editor and builds, and supports drawing from jobs.
Shapes is a paid alternative that is designed to draw good looking lines for editor and builds.