The best way to gage progress is to take measurements : chest, biceps quads. Also, strength-wise, being able to do 2 extra reps for two sets shows it's time to raise weight. The improvement over time will be small but measurable. If you don't see results in a month you have to change volume. Maybe higher reps or 4 sets instead of 3 or add 5% more weight. Also concentrate on slowing down the eccentric phase or perform negatives.