I wondered where that saying came from, especially after I learned what daisies mean to a lot of people.

I found this from Canadian Flower Delivery - "All About the Daisy Flower - If there a flower that is common and that everyone knows, it is the daisy. There are all kinds of great varieties of the daisy and people absolutely love them. It is probably just as common as the rose, but the cost is so much less and this is a great flower to give friends and family or to grow. Overall, people are a huge fan of the daisy and it is a great flower to give out to people you care about. With the daisy, it has a meaning that shows loyalty to love and commitment. This is a great flower to be giving out to those you truly care about." http://www.canadianflowerdelivery.com/daisy.aspx

Unfortunately, I couldn't find a real asnswer as to where the saying, "Daisies don't tell". comes from. It could relate to a little girl's game called 'he loves me, he loves me not.'