Go to Google Translate.com and type in chiens, chiennes. Then hit the speaker button in the lower right corner of the dialog box and listen for the clearly pronounced n in chiennes. Duo's n in chiennes takes a little more effort distinguish but when it isn't there you know it is masculine chien.
Their is a slight difference in "les chiens" and "les chiennes"; the first one ends with a more "ong"-like sound whereas the feminine form ends with a more "enn" sound. Also, they must be male dogs because blanc would become blanche (the first one ending with a "ong"-like or "onc" sound, whereas the feminine forms ends in an "onsh" sound).
'may have posted this comment too many times now, but as jeffers86 said: "meh..."