It's VERY easy to do. It's called context, interpretation and reading comprehension.
Think about the context of the current discussion, how it started and what's being said by which parties. When a certain comment that stands out seems like it's trying to make some sort of point but is rather exaggerated, look at what's being said, then repeat it again in your head to get a feel of how it'd sound if it was actually spoken. If that doesn't work, speak it out loud for yourself. You'll find that spoken out loud in response to the previous comment, in context of the conversation, it doesn't sound like it's being completely serious or agreeing with the other comment much at all in actuality. You can even try reading it in different tones if that's still not enough.
If you just look at it as nothing but words on the screen, that's all it'll ever be. Saying that it's "too hard" or that there being no faces or emotes makes it too emotionless is nothing but an excuse. You don't need those to understand sarcasm in a story or a satirical news article, so I don't see why you'd need it in a typical conversation on the internet. It's still a social conversation regardless of the method or media used for it. I can understand sarcasm on the net just fine as long as I'm actually using my brain at the moment or don't think too much. Sometimes, I do miss it. When I do, that's more than likely my fault and not the other person. It's not their problem that I couldn't identify their satirical wit, is it?
Anyway, that's my piece on that. I'm not attacking you or anything. I'm just saying that you and many others are just going about things the wrong way.
Ah, and I'm calling you a bunch of lazy whiners too.