when i did mine to make sure all was new i drained it. filled up with just water, ran for a min or two to get it mixed. drained again. filled with water, ran for a min to get it mixed, then drained a few letres out and put a few letres of coolant in to get the correct concerntration.
if there is any old stuff in there its seriously waterd down becuase the water drained looked like it was streight out of a tap, no dirtyness/ murkyness/ coolant colour etc.