When baking tangent space normals with Blender i had to fix the normal image by opening it with an image editor and inverting the red channel, to show correctly with Panda3D. I made a test with a torus baked to a plane and examined the color values in it which don’t match with the geometry.
A much quicker way to check a tangent space normal texture for Panda3D (is this different with other game engines?) is to open it with an image editor and examine it’s highlights in the three RGB channels. For the red channel (X) the highlights should appear to the right. For the green channel (Y) the highlights should appear bellow. For the blue channel (Z) this usually looks like a bumpmap.