How can I find safety standard authentication information about photocouplers?

You can check by using a parameteric search on the Toshiba website.

Also, you can check authenticated products from UL “Online Certification Directory” and VDE “VDE-geprüfte Produkte”.

EN 60747-5-5 is one of the typical component standards applicable to photocouplers. For products to comply with EN 60747-5-5 certification, a partial discharge test must be added to a usual shipment test. Toshiba thus operates EN 60747-5-5 certified products as follows:
(1)Assign uniform names to products to which the test required by EN 60747-5-5 has been applied, such as names unified as option (D4) or (V4) specifications.
(2) Assign uniform product names to products with (D4)/(V4) specifications.
(3) For product names, assign (D4) to package products for which a creepage distance/clearance of 6.4mm or more can be guaranteed such as DIP and SDIP packages; and assign (V4) to surface- mounted package products with a creepage distance/clearance of 5mm or less such as SO4, SO6 and MFSOP6.

Information related to safety standards is also provided in individual technical documents. However, for details, refer to Toshiba Application Note "Safety Standards for Photocouplers", Chapter 3. Safety standards.