How can I find the cutoff-frequency in real experiment?
It seems to be your task to figure out how to do this, and we don't solve homework for people. But your actual question here seems to be: I don't understand why I should find the method to determine cutoff-frequency.
The real life of an engineer involves not only designing things, but also verifying that your design works as intended (or more often, figuring out why it did not work as intended).
Imagine you have designed an ADC with an input filter, and the sampled data of the prototype is not what you expected. You measure the filter and realize it is 10 times higher than expected. A resistor is wrong, because manufacturing didn't get the errata you sent them, or someone took it from the wrong bin, or it wasn't soldered correctly.
Or a device that used to work comes back from the customer, they claim it doesn't work. They used it in a different country where it's colder, and you realize after verifying that you omitted to test the design over the whole temperature range promised in the manual, and it goes out of spec when it's too cold.
A lot of things outside the simulation can affect the performance of the product, and even if it's not your fault, being able to show the result on black and white goes a long way when figuring out the root cause.