How to know system requirement of an app?

If you're creating a regular app, like a feed, news or a browser, there's no need to specify a hardware (performance) requirement as such apps usually run fluently on a mediocre device.

If it's a game, the best would be buying a variety of devices with different specs and test them one-by-one, and decide the minimum requirement for the game to run fluently. Im case it's too expensive/unnecessary, you can run it on a device that you own, then compare its performance with a benchmark app like 3DMark or GFXBench. Look for benchmarks for other chips, and estimate the minimum requirement according to your game's test result and the benchmark results on different chips.