(I decided to turn my comment into an answer)
Get them to tell you a story.
The story of how they developed the piece, what the key elements are and why that is important.
Through the process of introspection and explanation both you and the designer could come to some valuable realisations. If the answer is something like "umm...because it just is". Then you might have cause for concern.
Design is storytelling. Design has purpose.
What is the design for? What personality should it have?
If you know what the design is trying to communicate and to which type of users it needs to relate, this should provide a frame of reference. Is it bold and loud? Professional? Caring? Does the design communicate these traits?
Do you want the icon to stand out in a crowded app store? Does it shape the customer perception in the desired way?
If you keep the discussion about business and user goals you reduce the risk of it sounding like a personal attack.
Designers can be their own harshest critic
Many good designers have found a way to be constructively critical of themselves. They needed to in order to improve their skills. To be at the top of their game.
The designer should present their work. Explaining how they arrived at the solution and how this meets the stated goals. Others can then ask questions or provide their views within this context.
If a designer can't elicit and work with feedback in a healthy way their value is limited. You will just be having the same battles over again.
Managers: If you hired a designer for their design expertise and the business and customer goals have been central to how you work, then you should place some trust in the designer. Ultimately, this can be backed up with data - test early and often.