determining how much data is on a socket

[Z 3 Penguin]

is there any way to find out how much data is waiting on a given socket?  my program reads 2 bytes from the socket to know how much more it needs to read.  however, sometimes it reads those two bytes and pauses until the rest of the data comes in.  i would like to know if the bytes are waiting before i read, so that if they are not, my program can continue and try to read it later.  any ideas?
-z