Strength training is about building and maintaining a certain level of strength. You might not be lifting enough weight during one or many of your exercises if:
1. The current weight you are lifting is not a challenge.
Strength training is meant to be challenging, because the whole point is to "overload" your muscles so they get stronger. If the weight you are lifting isn't as challenging as it used to be (or isn't as challenging at all!), then it is time to increase the resistance.
2. You could go forever.
Each strength training exercise you do should cause you to feel muscle "fatigue" within 15 repetitions (or fewer). Muscle fatigue feels like you couldn't possibly do another repetition in good form. If you can do more than 15 reps in good form, or if you literally feel like you could go forever because the resistance you're using is so easy, then it's time to take it up a notch.
3. You have never increased the weight you lift.
When you first started strength training, then the weight you lifted was a starting point. Continuing to progress in strength training is essential to getting the most out of your workouts--that means lifting more weight as you get stronger.
4. The progress has come to a stop.
Without making your muscles work harder than they're accustomed to, they won't get stronger. As you train, your muscles will grow stronger in order to meet the demands you are placing on them. So if you keep offering them the same workload, they will keep working the same amount, and progression comes to a grinding halt.
Recognize if you are experiencing any of the 4 signs above, taking time to pay attention to the level of difficulty and challenge of each of your exercises during your workouts. If you experience any of these signs, then it is time to increase the resistance!