If you never worked out before, then it is time!
But for the people who do, I will tell you here exactly how to know when to increase weight in your exercises based on my 5 years experience. I will tell you 2 tips, one mental and one practical, enjoy!
The Mental Tip
The only way to know how is to try go higher! Don't be afraid to get to new weights, one of the things that keeps us where we are and not letting us move forward is the lack of faith in yourself. Do not be insecure! Tell yourself you can. Imagine yourself doing it.
And I'm not talking only about gym here. This motto helped me in so many aspects of my life, whether it's with girls or work, or even on my ball team. In one year, I moved from being a bench player to be the captain of my team. But let's get back on topic.
The Practical Tip
Try adding 10 lbs to your weight first. If you can do 12 reps with GOOD posture, then you can go higher. If you can do at least 6-10 reps with GOOD posture, then stay with this weight on the next sets as much as you can, and if it's too much for you for the next sets, it's okay, go back to your previous weight and in the next workout try to complete 2 sets with the new weight.
This is the only way to know when you are ready, just "ask" your body.