Thai tones are never guessed. Every syllable in a written Thai word is accompanied by a symbol (called a “tone-mark”). A Thai word is written so that it is composed of letters (letters of the Thai alphabet) and “tone-mark” which mark every syllable in the word (the tone-mark indicates the tone of the syllable). There are five distinct tone-mark symbols. One of the tone-mark symbols indicate that a syllable is to be pronounced at mid-tone. The other tone-mark symbols indicate low-tone, high-tone, rising-tone, falling-tone. Therefore, a syllable must be pronounced at a tone level according to the tone-mark indicated in that syllable.

I presume that you already know that a Thai word means different things when that word is pronounced at different tones. Just to give you an example:

ข้าว = “kao” (falling-tone) = rice

ข่าว = “kao” (low-tone) = news

ขาว = “kao” (rising-tone) = white

The above Thai words when written in Thai script, will all have the same Thai letter sequence ข,า, and ว, but with different tone marks (those scrawny marks written above the syllable).

When you read a letter, the meaning is frequently altered by your perception of the “tone” of the letter. For example, if you think the writer is being sarcastic when he/she says “You have no idea how much I love you” the meaning to you will be completely different than if you think their words are sincere. Being able to figure out the tone of the written word is frequently very important to knowing the meaning.

Reading a speech instead of hearing it is another example.