1. An image forming apparatus comprising: an image forming mechanism configured to perform an image forming operation under predetermined image forming conditions, the image forming mechanism comprising: an image carrying member configured to form an electrostatic latent image thereon; a development device configured to develop the electrostatic latent image into a toner image; and a transfer device configured to transfer the toner image to a recording medium; and a control device configured to control an amount of toner replenished to the development device, to set a developer mixing time for evenly mixing developer in the development device, to cause the development device to mix the developer for the set developer mixing time, to cause the image forming mechanism to form toner image patterns, to detect the toner image patterns, and to determine the predetermined image forming conditions based on the detection of the toner image patterns.