Use three distinct checks. First identify grammar: is lā prohibitive or negative, and what governs the verb? Next determine the rhetorical force: prohibition, threat, supplication or entreaty? Finally test dependency: does the following clause express a consequence of the requested outcome?
Lā tashtimnī yakun khayran laka contains genuine restraint and a dependent jussive response. In the threatening lā tamtathil amrī, syntax can be similar while the intended effect changes because of the context.
StepQuestionConclusion
1: SyntaxWhat kind of lā and verb?Prohibition/negation and jussive
2: RhetoricWhat is the speaking context?Restraint, threat, prayer or request
3: LinkWhat makes the consequence possible?Request-response or unrelated clause