StoreKit on iOS - autorenewable subscription - How to know whether it is a free trial or not?
TLDR; It is not possible. You need to manage this information yourself.
When you process an SKPayment you get back a receipt. You should verify those receipts regularly for subscriptions (e.g. before downloading new content) as the user might have cancelled the subscription. When the user does cancel the subscription or the subscription expires you get back a descriptive error when verifying the purchase's receipt.
Moreover the receipt gives you all the information you need: for a given product you know which trial period you grant. Therefore when a purchase is made you could store the purchase date given from the purchase receipt in your model object or in the NSUserDefaults or in the Keychain alongside the purchase data. At this point you know when the trial is expired and verify that the subscription is still valid. If you can't you might want to disable access to the content until you're able to do so.
For more informations about purchase receipts and subscriptions check out the In-App Purchase Guide by Apple.
On the client side you usually identify the different products and characteristics by their product identifier as the App Store does not deliver certain information such as subscription period and free trial period.
So if your product ID is for example:
com.domain.app.product_paid1month_free7days you split the ID on the client side and know that the paid subscription duration is 1 month and the product has a free trial period of 7 days.
Of course one approach would be to transmit the product ID to your own server to get its characteristics as response. This way you can maintain the product list continuously without updating the binary and across versions.