Firstly, you can consider your heartrate. The resting heart rate of an adult is normally between 60 and 100 bpm. Secondly, you can check your weight. The body mass index measures body fat in adults: the healthy range is between 18.5 and 24.9. Thirdly, think about your sleep patterns. Can you fall and stay asleep, and do you feel well rested?
Finally, however, the best way to pay attention to your health is by visiting a doctor for annual check-ups or if any concerns arise.