How can I tell if it's a Twitch Prime subscription or just a subscription?

You don’t/can’t

At time of writing EventSub doesn’t state a user (re)subscribed with Prime.

There are a number of uservoices in from myself and Soaryn to improve the whole of EventSub broadcast susbcriber information, including but not limited to “user used prime”

This is Soaryn’s on the matter Revise the `channel.subscription` topics – Twitch UserVoice