Maybe 85% of detecting sarcasm involves knowing the speaker's actual opinion on the topic so the listener can assess the low probability of earnesty.

The other 15% is present in the overdirect phrasing of the counterintuitive opinion.

So, where a low probability of a speaker's earnesty is present: if they employ very clear verbiage, and especially emphatic punctuation or enthusiastic modifiers, it is more likely sarcasm than some other alternative (e.g., changing opinion or acknowledging nuance).

Clarity provides a maybe 60% chance where counterintuitive enthusiasm increases to near certainty.

A vegan about a steak:

I. "Doesn't that look delicious!"

V.

Ii. "That actually kind of looks delicious...."

Both are counterintuitive statements of the speaker, but the emphasis and certainty of statement i. versus the uncertainty present in option ii. makes it clear which is more likely sarcastic.

Great question, by the way!