how do i check if my rigidbody player is grounded?

Aldo Naletto's idea's really good, I've been using this in my game for about 2 months. But there's one small flaw. If the ground is a bit uneven, it doesn't really work. "I'll cast the ray further then" was my first idea, but what if it's a hole? What is needed is a capsulecast instead of a raycast:

 //collider.bounds are the bounds collider relative to the world. I wanted a 0.1 margin, and 0.18 is the radius of my collider.
 Physics.CheckCapsule(collider.bounds.center,new Vector3(collider.bounds.center.x,collider.bounds.min.y-0.1f,collider.bounds.center.z),0.18f));

 var isgrounded : boolean = true;
 
 function Update()
 {
     if(isgrounded == true)
     {
         //Do your action Here...
     }
 }
 
 //make sure u replace "floor" with your gameobject name.on which player is standing
 function OnCollisionEnter(theCollision : Collision){
     if(theCollision.gameObject.name == "floor")
     {
         isgrounded = true;
     }
 }
 
 //consider when character is jumping .. it will exit collision.
 function OnCollisionExit(theCollision : Collision){
     if(theCollision.gameObject.name == "floor")
     {
         isgrounded = false;
     }
 }

this works for me quiet well.....

Why don't use Unity's OnCollisionStay function?

I've used "Raycast down" method like aldonaletto suggested but it causes issue when player stand on an edge and the raycast miss the edge.

Understand how OnCollisionStay works

With OnCollisionStay, you can accurately detect the ground. Here is how OnCollisionStay works:

 using UnityEngine;
 using System.Collections;

 public class ExampleClass : MonoBehaviour
 {
     void OnCollisionStay(Collision collisionInfo)
     {
         // Debug-draw all contact points and normals
         foreach (ContactPoint contact in collisionInfo.contacts)
         {
             Debug.DrawRay(contact.point, contact.normal, Color.white);
         }
     }
 }

Attach the script above to your character (your character has CapsuleCollider component), then run the game, look at your character in Scene widow instead of Game Window (because Debug.DrawRay only works in Scene window), you should see a white ray where the character touches things.

Implement the idea

 //----------PlayerCollider.cs------------
 public class PlayerCollider : MonoBehaviour
 {
     private bool m_IsOnGround;
 
     public bool IsOnGround
     {
         get
         {
             if (m_IsOnGround)
             {
                 m_IsOnGround = false;
                 return true;
             }
             else
             {
                 return false;
             }
         }
     }

     void OnCollisionStay()
     {
         //If it touch things, then it's on ground, that's my rule
         m_IsOnGround = true;
     }
 }
 //----------Player.cs------------
 public class Player : MonoBehaviour
 {
         PlayerCollider m_playerCollider;
         void Start()
         {
             //...
             m_playerCollider = GetComponent<PlayerCollider>();
         }

        void Update()
        {
              if (m_playerCollider.IsOnGround)
              {
                    Debug.Log("On Ground");
               }
               else
               {
                     Debug.Log("In air");
                }
         }
 }

Want a working example out of the box?

Check this free asset: Ragdoll and Transition to Mecanim (the time I write this answer, the asset is free, guess it will still be free by the time you check it)

The character controller in the asset above uses OnCollisionStay to detect ground.

Is it possible to get aldonaletto´s answer translated into c# ?

I tried to translate it right off but that didnt help. I am new to java and c# so please dont flame me.

I did this but get some errors like:

Operator &&' cannot be applied to operands of type bool' and UnityEngine.Vector3' The best overloaded method match for UnityEngine.Physics.Raycast(UnityEngine.Vector3, UnityEngine.Vector3, float)' has some invalid arguments

Argument #3' cannot convert double' expression to type `float'

 // Jump variable
 private float distToGround;

 if(jump_key && IsGrounded())
 {
 Jump();
 }
 
 void Start () 
 {
     //Get the distance to ground.
     distToGround = collider.bounds.extents.y;
 }
 
 void Jump()
 {
     rigidbody.AddForce(Vector3.up * jumpForce);
     animation.Play("jump_pose");
 }
 
 void IsGrounded()
 {
     return Physics.Raycast(transform.position, - Vector3.up, distToGround + 0.1);
 }