I'm afraid my wily answer is I don't know how I differentiate - it's one of those parts of your own speech pattern that you can't hear and that disintegrates when you try to examine it. I'll give it some more thought tomorrow when my brain's less tired! Mebbe some of the others will have a clearer take on it whilst I am indisposed.

edit: I see my difficulty in discriminating is not without foundation!

http://www.worldwidewords.org/qa/qa-whi2.htm