It may be worth mentioning that the Irregular Tone Rule* (e.g. the นน in ถนน unexpectedly adopting a rising tone, by 'inheriting' the class of the leading ถ ) only applies in cases where the ะ is not written. So, if you hear a word that fits this pattern, but the tone of the second syllable is not what you'd expect, you can infer that the ะ is not written. This won't solve all your problems, but it narrows the field a little. * More on the the Irregular Tone Rule at the excellent 'Inside a Thai Syllable': http://www.thai-language.com/id/830222 at section IIIb, heading, "b.