In the Subscription you can check the "Activity log" section for all actions on the subscription. If we're talking about a new subscription, not resources inside a subscription - Azure AD Audit logs provide only update & delete operations as far as I know.
With regards to prevention of free trials - there was a similar question asked 9 months ago here - https://learn.microsoft.com/en-us/answers/questions/223759/prevent-trial-subscriptions-being-created-in-tenan.html
Same post will point you to the following MS Learn article on preventing trial subscriptions. https://learn.microsoft.com/en-us/microsoft-365/commerce/subscriptions/allowselfservicepurchase-powershell?view=o365-worldwide#requirements