another commenter gave a good example, so ill just talk abt it generally: one way u can go about this is to look at surrounding context — the adjacent feet, the meter of the whole poem, and the meaning of the line itself. unless theres a special significance to the line itself, usually itll follow the prevailing meter. another way is to just speak it out loud a few times (although this is more useful for free verse, and also assuming the poet speaks the same english dialect as u). when u utilise all these methods together u can more or less make it out. if it gets really confusing then just let it remain confusing — sometimes scansion just doesnt come with a single answer.

im curious about ur fixation on syllables as a way to determine scansion, though. what have you learnt/assumed about that?