Colliding = Distance ≤ r₁ + r₂ = =

Get distance from one circle's center to other circle's center.
√(x₁ + x₂)² + (y₁ + y₂)² = d, this is usually (vec1 - vec2).magnitude in programming.
then, check if the value is greater than or equal to sum of it's radius.
if it is, then it is colliding.