Beyond the tone and expression sarcasm can often be identified by context.
Generally when people say very positive things about very negative situations they're being sarcastic. As in:
I love it when my car breaks down.
Thanks for dumping all your tedious work on me, I really appreciate it.
Sometimes tone and expression aren't enough to identify sarcasm, because some people have a very flat delivery style which can be used to accentuate the comedic value of the statement. Some comics play this deadpan angle to great effect, think Tig Notaro, or Stephen Wright.