A neutral-tone syllable is unstressed and normally has no pinyin tone mark.
Notice what is written#
An unmarked syllable in a pronunciation guide may indicate neutral tone. A casual text that omits all accents is not enough to determine the tones.
Suggested practice#
Use the linked course audio to compare an accented syllable with an unmarked one. Listen to the full word or phrase. Do not replace a recording with a rule that every neutral syllable must have one fixed pitch.
Reference#
Pronunciation reference. The practice instructions here are newly written. Recordings, charts and illustrations from the reference are not reproduced.