You’re probably already aware of the naming conventions used in the original WADs. Search for one of this terms and you’ll find textures corresponding to the category:
- generic: self-explanatory (vending machines, breaker unit, special glass...)
- glass
- w, wall
- out: outdoor-related
- flr, floor
- lgt, light
- ceil, ceiling
- crate, box
- btn, button
- trim
- dr, door
And a few others I forgot.
I would advise just leisurely scrolling through part of the wad when you’re looking for a texture (not necessarily the entire WAD). This will help you each time get a better glimpse of the available textures.
Certain mods like Blue-Shift excel in using the original HL textures in an elegant way. Other mods use them in a very original way for which they were not intended. My second advice would be to play maps and mods and take screenshots and good texturing choices.