A rational number is simply a term that can be expressed as a fraction. Otherwise, that is an irrational number. So, you can use a calculator to verify if the number is rational or not.
The key characteristic of an irrational number is when it contains a long line of decimal places. For example, the term π and the Euler's number e are irrational numbers. The exact values of π and e are 3.14159 and 2.71828182846, respectively. In reality, those decimal places go on a long way. Particularly, π has a total of 2.7 trillion digits. Numbers inside radicals or roots can also be irrational numbers. For example √3 is irrational because it is equal to 1.732050808. However, not all radicals are irrational. For example √15.3664 is equal to 98/25 or 3.92. That is a rational number. So, therefore, use the calculator to know the exact value of the term to properly distinguish rational from irrational.