Sorry, there is no way to check if skus is a trial or paid, this feature is not available yet, you can see the list of property available for Skus - https://learn.microsoft.com/en-us/graph/api/resources/subscribedsku?view=graph-rest-1.0#properties
As i already saw couple of same questions, if you want this feature to available, you can raise a feature request - https://feedbackportal.microsoft.com/feedback/forum/ebe2edae-97d1-ec11-a7b5-0022481f3c80
Hope this helps
thanks