It depends...sometimes it was obvious while Tyler was awake. He'd be grumpy, clingy, or chewing on his hands a lot. Other times, he'd be perfectly fine, but sleep would be wacky. That's when I'd bring out the Motrin or Tylenol...if either of those made a difference in his sleep, then I could usually assume teeth were bothering him. If not, I knew it was something else.
Of course, sleep and eating always go crazy right before Tyler gets sick, so be on the lookout for those symptoms as well. HTH!