Hi,

You can get it from the table ir.model.data by searching using the id and the model name, suppose if you have to get the external ID of the record with ID 10 in the model res.groups, search like this,

domain = [('model', '=', 'res.groups'), ('res_id', '=', 10)]
model_data = self.env['ir.model.data'].search(domain, limit=1)
xml_id = "%s.%s" % (model_data.module, model_data.name)

To see this table in the UI activate the debug mode and navigate to Settings -> Technical -> Sequence & Identifiers -> External Identifiers

You can see similar example in the odoo code itself, see the _get_external_ids function inside this file https://github.com/odoo/odoo/blob/13.0/odoo/models.py

Thanks

You can browse record and call the function  get_external_id()

xmlid = record.get_external_id()

Using this answer from Ray Carnes this code works for me in Odoo 13:

https://www.odoo.com/es_ES/forum/ayuda-1/how-can-i-show-a-record-s-external-id-in-a-list-view-i-also-want-users-to-be-able-to-search-for-records-using-this-field-146080

product_id = fields.Many2one('product.product', 'Product')
prod_ext_id = fields.Char('ID Ext Product', compute='compute_ext_id_prod')

def compute_ext_id_prod(self):
    for record in self:
        res = record.product_id.get_external_id()
        record.prod_ext_id = False
        if res.get(record.product_id.id):
            record.prod_ext_id = res.get(record.product_id.id)