You still need a minimum level of intensity. You won't be improving strength with something that you can lift 50 times.
One way to measure perceived exertion is how many additional reps you think you can do - if you're doing 10 reps at a weight where you can only do 10, then you're obviously at some risk of having worse form / injury; if you're doing 10 reps at a weight that you can lift 12-15 times, you'll probably get stronger, if you're doing 10 reps at a weight you can lift 25-30 times, you likely won't make a lot of progress.
One way to measure perceived exertion is how many additional reps you think you can do - if you're doing 10 reps at a weight where you can only do 10, then you're obviously at some risk of having worse form / injury; if you're doing 10 reps at a weight that you can lift 12-15 times, you'll probably get stronger, if you're doing 10 reps at a weight you can lift 25-30 times, you likely won't make a lot of progress.