Is there a way to tell the difference? From what I read Lilacs are kind of rare. I would hate to butcher a rare bird.
I believe that the easiest way to differentiate them is to check the newly hatched poult color.
http://www.porterturkeys.com/lilac.htm
http://www.porterturkeys.com/variouscolorcombos.htm
I suspect that a self blue rusty slate would have a newly hatched poult that is a lighter version of the rusty slate poult.
I also think that the tail feathers of a lilac are not as dark as those of a rusty slate but your question is best answered by Mr. Porter. There is a contact button on the main page. They also have a turkey genetic group that is good at these answers.