I wouldn't say I'm particularly sarcastic, but in online conversations I tend to punctuate a lot of my statements with the ':P' symbol to show that I'm not entirely serious about whatever I just said. It's more effective on MSN messenger since that symbol is kind of a goofy smiling face.

I have trouble detecting sarcasm from people online that I don't know. I've learned not to overestimate the intelligence of the anonymous masses.