Function to detect trigger recursion

Hello:

Is there a builtin mechanism for detecting trigger recursion or more
specifically
one trigger , causing another tables trigger to fire.

Example:

Table A is updated

Table A's update trigger executes
{
    Updates the B table - (maintaining special RI etc.)
    Table B's update trigger executes
     {
           <does some stuff>
           <need to know if I was triggered from within another trigger> ?????
( Is there a function that does this? )
           <does some more stuff>
      }
}