To find out the network type, run this command with credientials of a user with the admin role:

$ quantum net-show <name or id>

and look for the provider:network_type attribute. Regular tenants will not be able to view this attribute.

It is not possible to determine the network type (or quantum plugin) from an attached instance. Also note that the OVS plugin can be configured to provide networks of type gre, vlan and flat in the same configuration - ie they are not mutually exclusive.