if my onprem AD group is synchronized to Azure AD will i be able to make the onprem AD group as a member of cloud DL.
Yes, you could add synced security group into a Exchange online distribution group.
Also how do i check which of my OUs are synchronized to Azure AD
Here are detailed information about it: Select the domains to be synchronized using the Azure AD Connect wizard
You could manage synced OU from AAD Connect wizard:
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