If you are consuming a JWT, you can see if a user has enabled two factor authentication by putting a claim in the JWT using a populate lambda. Look at the user object and if the twoFactor.methods array isn't empty, they have enabled MFA. If you are not using a JWT but instead examining the user object directly, you can look at the same attributes.
In each case, you should set up a page to allow the user to enable MFA and keep directing them there until they have done so. You can either build your own 'MFA enable' page or, if you have a paid edition, use the themeable account self service pages, as documented here: https://fusionauth.io/docs/v1/tech/account-management/