Question

how can I understand that the number is rational or irrational?

Answer 1

To identify whether a no is rational or irrational look at its decimal expansion. If the decimal expansion is non terminating non repeating than the no. is irrational.
Also if the no. can be expreesed in the form p/q where p and q are integers and q does not equal to 0 than no is rational otherwise irrational.

Answer 2

if the number is integer
means – 0,1,2......-1,-2
and can be written in the form of fraction
(p/q and q not equal to zero) is rational .

note that any number In the form of root is not rational but numbers like √4 can be written in rational form because √4=√2*√2= 2

numbers which are non terminating and non recurring is also consider irrational
example– 0.20200200020002......