A quick and dirty (though not foolproof) way of doing this is by looking for key structural indicators in the stimulus (e.g. thus, therefore, so, hence, it follows that, establishes that, etc.) which point to the introduction of the argument's conclusion. The reason I say this isn't always foolproof though, is that sometimes these structural indicators are used to introduce subsidiary conclusions, which function to support the argument's main conclusion.
I think the best and truly easiest way is to actively read the stimulus for support. As you read, ask yourself how the stimulus is using each sentence--does the sentence/idea function to support some other idea? If so, then it is not the conclusion. The main conclusion of the argument is always a statement/idea that is supported by virtually everything else supplied in the stimulus, and it doesn't support any other idea.
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I think the best and truly easiest way is to actively read the stimulus for support. As you read, ask yourself how the stimulus is using each sentence--does the sentence/idea function to support some other idea? If so, then it is not the conclusion. The main conclusion of the argument is always a statement/idea that is supported by virtually everything else supplied in the stimulus, and it doesn't support any other idea.