This is the easiest way to tell if you are actually taking reps close enough to failure.
Compare the SPEED of your First rep and your LAST rep.
If it is not significantly slower, then you are not reaching that lovely threshold that actually causes adaptations in the working muscle(s).
You don't have to take it to complete failure, but within 1-3 reps is that sweet spot you want to hang out in for TWO of your working sets.
Hope this helps