are .tga files always reflection maps? and conversely, are reflection maps always .tga files?
no. tga is just a file format for images with an alpha channel.
and as wolli said: the reflection map can be a dds file (or of any format which features an alpha channel and is supported by morrowind).
so unfortunately you cannot designate the ref map with it's file format.
the easiest way is to open one nif in NifScope and take a look at the texture properties. most likely similar meshes (interior meshes in your case) will use the same reflection map.
if you're unsure, just open the textures and take a look at them. reflection maps most likely don't look like actual textures. most ref maps i know look more like greyscale patterns on solid black or transparent ground (depending on which program you use to open them).
you could try http://www.btinternet.com/~mnwright/programs/dxtbmp.htm to open them. it supports TGA, DDS and even DXT compression.
if you're still unsure, backup the textures and replace one by one with a dummy and look ingame if the weird reflection is gone.
if it's still there, it is the bump map, like AOF said ( and in this case i can't help you either
)