oops! sorry about that!
the ABS is harder and less malleable.
heres a quick, easy way to test it: bend one of the filaments 180 degrees. the ABS will generally turn even more white. also, it will bend less easier.
if you dont mind putting some in your mouth, try biting it... the HDPE will be much easier to bite than the ABS.
of course there is a 'scientific' way... google plastic identification and you will find a flowchart of various tests you can do to identify a polymer.