Always ask how the patient feels. Always listen to that, and take some cues for further assessment from it. In the case of fatigue, the obvious choices are anemia, hypovolemia, and CHF, but electrolyte imbalance, sleep deprivation, some medications, and hunger will do that too.
Subtle changes-- look at preop VS; you can find them in the holding area charting. VS may be "normal" but not normal for this patient. The classic example is the athlete whose baseline resting heart rate is somewhere in the 50s, or even less, with a BP in the 100-110 systolic range. If postop his heart rate is 80-ish and his BP is 104 systolic, who's gonna worry? You are, if you're careful. He has increased his heart rate by 60% (and he has a big athlete's heart so that's a big increase in cardiac output) but his BP hasn't gone anywhere. Danger, danger, Will Robinson ...
You also want to look at trends. A very teeny, small change repeated every hour adds up to a big one, and you might not think much of it unless you look at the whole sequence of events. An SpO2 that starts at 99%, an hour later is 98%, an hour later is 97%, then 95%... all normal, but dropping. Who remembers to look back at four or six hours ago? You do, that's who.
A BP that creeps down, a HR that creeps up, a resp rate that creeps up (and we know that nobody really counts RR, but we also know how wrong that is) -- all of these can be telling you about compensation mechanisms starting to fail. You don't want to see those in retrospect with a forehead-slap moment, you want to maintain a decent index of suspicion from the beginning.
This isn't necessarily a post-op-specific tip, but if your patient ever tells you he feels like he's going to die, or even that "something's not right," pay close attention. A lot of the time when someone says that, he/she is right.