if it can be expressed as a fraction, it's rational.
so if it's just 4.7010010001, it's rational.
because the fraction of that is 47010010001/10000000000 (yucky, i know, but it's still a fraction)
if it's a surd (like sqrt2), it's irrational
or if it's things like pi, e, or other non-recurring numbers that go on for infinity, it's also irrational.