This may seem like a silly question: How do I know if I am doing my best?
we are always doing our best. What "best" means changes over time as our capacity increases (or decreases). There's a difference between doing our best, and "being" the best, or even giving 100% effort 100% of the time (which is impossible). What's important is not trying to be doing our "best", but just staying consistent at putting in whatever effort we can.
An example. Let's say you decide to run for exercise and to lose weight. At first you might only be able to walk at a pace of 20 minutes a mile for a couple of miles twice a week. But if you stick with it, over time, let's say in a year or so, with the same effort, you can now jog at 10 minutes a mile for 4 miles, 4 times a week. You've doubled everything with the same relative effort as when you started. In fact it was probably harder when you first started.
The point is, there's not a competition. Best is a meaningless concept. Be who you are, in the moment. It like that scene in Apollo 13 when they fixed the air filter and they are waiting to see if it works and Tom Hanks says "Just breathe normal fellas."