My ex and I have a child custody consent order stating we have joint legal custody.

However, my ex took our child for a mental health evaluation with a child psychiatrist behind my back. He never told me or consulted with me about this. I found out about this through her medical insurance company.

Is he in contempt of our order? What can i do?

If your order states that you have the right and obligation to consult and be consulted on all major medical decisions affecting the child there is a good chance your ex will be held in contempt.
While you certainly cannot reverse his actions you can ask that a judge hold him accountable for his behavior and admonish him not to take unilateral actions in the future.
To do this you will need to file a motion to show cause. The clerk will then issue an order for your ex to appear in court and show cause why he should not be held in contempt based on his actions. You will also need to appear at the hearing and present your evidence to the judge.

The order is not that specific. All it states is that we have joint legal custody.

Sorry I did not finish my response.

The order just states the parties have joint legal custody. My understanding is joint legal custody is that the parties together are to make medical and legal decisions concerning the minor child.

Since my order simply states that the parties have joint legal custody do I have the grounds to persue contempt.

Yes, the general meaning of joint custody is that you have the right to be consulted on major decisions involving the children, as well as the obligation to consult with your ex when the child is with you.