How to determine whether T is a value type or reference class in generic?

I have a generic method behavior of which depends on T is reference type or value type. It looks so:

T SomeGenericMethod <T> (T obj)
{
  if (T is class) //What condition I must write in the brackets?
   //to do one stuff
  else //if T is a value type like struct, int, enum and etc.
   //to do another stuff
}

I can't duplicate this method like:

T SomeGenericMethod <T> (T obj) where T : class
{
 //Do one stuff
}

T SomeGenericMethod <T> (T obj) where T : struct
{
 //Do another stuff
}

because their signatures are equal. Can anyone help me?