identifying a subscription product in the cart to calculate shipping cost

Hello,

I would like to be able to identify if a product in the cart is a subscription so that I can offer a varied delivery cost. Currently, all subscriptions gets free delivery, one off gets charged. I can charge dependent on the user role, but for a new customer who has not yet checked out and registered there is no way of knowing that a product in the cart is a one off or a subscribed product.

Change the shipping class if the product is a subscription?

Thanks

Mark