Checking if float is equivalent to an integer value in python

In Python 3, I am checking whether a given value is triangular, that is, it can be represented as n * (n + 1) / 2 for some positive integer n.

Can I just write:

import mathdef is_triangular1(x):    num = (1 / 2) * (math.sqrt(8 * x + 1) - 1)    return int(num) == num

Or do I need to do check within a tolerance instead?

epsilon = 0.000000000001def is_triangular2(x):    num = (1 / 2) * (math.sqrt(8 * x + 1) - 1)    return abs(int(num) - num) < epsilon

I checked that both of the functions return same results for x up to 1,000,000. But I am not sure if generally speaking int(x) == x will always correctly determine whether a number is integer, because of the cases when for example 5 is represented as 4.99999999999997 etc.

As far as I know, the second way is the correct one if I do it in C, but I am not sure about Python 3.