How do I check if my player is grounded with physics?

I try to make player jump with Rigidbody but it doesn't work.

 checkCollider = Physics.OverlapSphere(groundCheck.position, groundRadius, groundLayer) // a collider with its parameters
 if(checkCollider.Lenght > 0) {
 isGrounded = true;
 } else { 
 IsGrounded = false:
 }
 if(Input.GetKey (KeyCode.Space) && isGrounded) {
 GetComponent<Rigidbody>().velocity = (GetComponent<Rigidbody>().velocity.x, jumpSpeed, GetComponent<Rigidbody>().velocity.z);
 // I have the jumpSpeed variable