There are a couple ways to distinguish a trial key from a subscription key.
License Model Method
As of QLM 9, a new property can be associated to a license called LicenseModel. LicenseModel can be set to one of these values: trial | subscription | permanent
When you create the license key, you set the LicenseModel property. This can be done from the QLM Management console, from an API call or from an http method.
When a license is activated via an API call (ActivateLicense), the returned xml fragment contains the license model property. After calling ParseResults on the xml fragment, you can access the license model via the ILicenseInfo.LicenseModel property.
Another approach consists of creating a feature for your product called TRIAL (on the Define Product page, select your product and create a new feature called TRIAL).
When you create a license key, enable or disable the TRIAL feature as required.
After you validate a license key, call the IsFeatureEnabledEx API to determine if the key is a trial.