An Antivirus vendor detects XYZ virus. You want to know if Symantec's Antivirus products detect it.
Symantec can not assure with 100% certainty that a threat is detected by the Symantec Antivirus products (Symantec Endpoint Protection and others), even if it is acknowledged as a common alias in the "Also known as" field in a virus writeup. It is a common practice in the Antivirus industry to put different strains of the same virus under the same name. So, even if Symantec recognizes that w32.virusname.variant is also known as XYZ by another vendor, we may not detect all the variants that other vendor detects.
If you want to check if a virus detected by a competitor is mentioned in the Symantec writeups:
Go to the Symantec home page
In the search text box type the name of the virus provided by another vendor
In the drop down menu select "Viruses and Risks"
Click on Search
If the virus is mentioned in any of the Symantec public writeups, you will see it in the results page
This does not guarantee that the particular variant that the other vendor detects is detected by Symantec, as some vendors put different strains of a virus under the same name.