Wildfire marks a block as flammable if either of these cases are true:

• It includes the flammableDamage table.
  
• It uses one of the wood footstep sounds (footstep_wood or footstep_lightwood).

The latter case works as a fail-safe method and allows the mod to also pick up blocks which do not include the flammableDamage table erroneously (i.e. cabin roofing).