How to know if trial is verified?

I want to display dialog asking some information if user is not verified trial? How to know, user has verified trial?

You know whether its verified or unverified because youre passing the TA_VERIFIED_TRIAL or the unverified version of the flag to TA_UseTrial().

See: https://wyday.com/limelm/help/trials/

Dont mix and match verified and unverified trials. Use one or another.

What I want to know is whether I have called the API 'TA_UseTrial()' before or not? I saw the exception 'AlreadyVerifiedTrialException' in TurboActivate.cs file, but is never thrown.

You control whether a trial is verified or unverified. And thus there's no API in TurboActivate for determining it. You control it thus you know it.