I can usually understand irony and metaphors (although, it can vary, sometimes I miss metaphors if they're subtle) but sarcasm is a different matter entirely. Admittedly, I am prone to missing sarcasm and taking things literally instead, despite this I sometimes use sarcasm myself so clearly I understand what it is... I just miss the signs now and then.
Generally I find it easier if I already know what someone's stance on an issue is, for example if I know someone hates spiders and they suddenly remark "I just love spiders!" then I know it's sarcasm based on deduction. However, if I don't know them that well and they say this then I'll probably reply "Really? Personally, I hate them" and miss the sarcasm (unless it's fairly exaggerated). If I know that someone is often sarcastic, then I will purposefully look out for it more and have a greater chance of spotting it.
When I'm sarcastic, I often exaggerate my movements. I use jazz hands to show that I'm not being serious, and sometimes roll my eyes or give off a slight laugh. Although I also use jazz hands to represent a light-hearted mood in the conversation, regardless of whether or not it's sarcastic...it's kind of my way of letting others know that what I'm saying isn't too serious and shouldn't be taken as such. 
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Near the spectrum but not on it.