Yes, you can guess. But assessment depends strongly on context. And Pow's Law still applies. People can write messages that seem sincere, and then later claim sarcasm. As in "I was only joking". Or people can write sincerely, but come across as sarcastic, or vice versa, and yet be ambiguous enough that readers can't tell. That's where the /s flag help. Done intentionally, such ambiguous messages can probe the reader's state of mind. Or set traps.

But maybe you're just being sarcastic ;)