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It is pretty straightforward to check a MySQL / MariaDB table for a Primary Key, connect to the db server, login or open up your favorite MySQL client.
Without selecting any table, issue the following command:
SHOW INDEXES FROM <TABLE> WHERE Key_name = 'PRIMARY'
Where <TABLE> is the name of the table you want to check.
The result returned should have a line show up with the Primary Key.
Note: If the result is empty, that means the table does not have a Primary Key.