How do I determine the actual amount of power my nuclear reactor can provide?

Water and steam bottlenecks will not become apparent until you actually start using the water and steam. So I don't think you will be able to find out the practical limit without drawing the power.

That said, you do not have to actually use all that power to test the output:

  • If you have large banks of accumulators, recharging them provides plenty of load on your grid.

    A single accumulator can draw only 300kW, however, so it will take 134 of them to accommodate a single 40MW reactor (no adjacency bonuses) - not practical if you do not already have large banks of accumulators.

    If you do have enough, simply disconnecting power until they are discharged and then reconnecting it should show you the actual power output of your plant.

  • Alternatively, placing a bank of fluid tanks behind your steam turbines will allow them to absorb all the excess steam production which can actually reach them.

    Grab a timer, connect the fluid tanks, wait for a time period you're comfortable with, then disconnect them. Each unit of steam has 0.097MJs of energy, so amount_of_steam * 0.097 / time_in_seconds will be equal to the excess output of your plant.

    Since this is the excess output, add it to your actual consumption during the period to get the full output.

    (Output capped at the theoretical turbine output, 5.82MW * turbines).