You can make use of Postman API to generate the token and you can find expiration time in the response along with token like below:

To validate the auth token, you can make use of JSON Web Tokens - jwt.io.
- After generating the auth token, you can paste the token in the above link and decode it.
- Usually, the decoded token has sections like below:
- header: It includes
alg which specifies the type of algorithm used to digitally sign the token.
- payload: It includes information about audience, scopes, expiration details, app details etc.
- verification signature: It includes the digital signature of the token that was generated by Azure AD’s private key.
To check the token expiration, you can find expires_in variable under payload section of decoded token.

You can check the below references to know more in detail:
Validate Azure Active Directory (AD) generated OAuth tokens (voitanos.io)
How to verify token in Azure Active Directory – tsmatz (wordpress.com)