If you are using AADConnect to sync from on-prem to ExO, thats expected. It can take up to 30 minutes to sync unless you do a manual delta sync.
If you want to simply make sure the mailbox exists in 365, get-mailbox <user> will return that object when connecting to Exchange Online via Powershell.
Unless you are thinking you need to check something else to verify the license.
If so, then:
Get-MsolUser -UserPrincipalName <user account UPN> | Format-List DisplayName,Licenses
Get-MsolUser -UserPrincipalName <user account UPN> | Format-List DisplayName,Licenses