If you really mean "How do I know my 20A breaker will really cut out at 20.1A?"... the answer is: it likely won't.
What the 20A rating on a breaker means is that it will deliver 20A indefinitely without damage. There is a document from the manufacturer called a trip curve that shows the design spec for your breaker. The curve and documentation will tell you what amperage you can expect your breaker to actually trip at. For example, my SquareD QO 15A single pole breaker has a curve that specs that for a load of 45A, the breaker will trip some time between 3 and 9 seconds (at ambient temp of 40deg C). The NEMA standard (according to SqD) for this breaker is that it MUST trip within 100 seconds for that load (at 25deg C). SqD also states that the breaker WILL trip at 1.35x rating (according to the curve, this occurs somewhere between 50s and 1000s). (this is in the thermal zone for the breaker)
If you mean: "How do I test that my breaker will trip in overload?" Well, just put that minimum load on it and see. So, for my 15A breaker, this means place 20.25A of load on it, make sure it stays on for 50 second and then wait the remaining 1000 sec (i.e. 16min 40s). If it trips at any point within that time, then it is working within spec.
If you mean: "How do I test to be SURE my breaker will trip on a short circuit?"... the answer is... without fairly expensive equipment you can't.
I'd say it is definitely out of the range of average DIY to safely test and correct interpret the results of a test. 'short circuit' is in the magnetic zone for the breaker, and starts no sooner than 7x the rating (140A) and no later than 15x the rating (300A) for my breaker. At some point near 15x (300A), the breaker is expected to trip within a single cycle.
If you mean 'How can I test the GFI/GFCI/AFCI portion of my breaker for proper functioning?'... the only accepted method by NEC is 'push the test button'. There are cheap GFI/GFCI testers and there are expensive GFI/GFCI testers and there are expensive AFCI testers - NONE of which are a valid means of testing, by NEC.