If the school doesn't feel there's a problem, or if they say something like "just give your daughter time to grow up (or catch up)," you have the right to insist that they do an evaluation -- and they have to do it! If you and your ex-wife don't have the kind of relationship that will allow you to work together on this, your ex can make contact with the school, and then request that they keep you informed. If your daughter's mother doesn't agree with you, then you can have your daughter evaluated. It's obviously better all the way around if you're both in agreement about this, since your daughter may get caught in the middle of your disagreement, and that could make her learning problems worse. It's also possible that your little girl's difficulties in school are not due to learning disabilities, but may be caused by her emotional reaction to your divorce or the conditions that led up to it; make sure you look into this possibility.
For an evaluation of learning disabilities outside the school, you can contact a psychologist, assuming you have the legal right to take this step. You may want to re-read your divorce agreement, or check with your attorney. You will want to locate someone who specializes in the evaluation and treatment of children with school-related problems. You can call the State Board of Registration of Psychologists for a recommendation. You can also contact the Learning Disabilities Association of America (LDA) to get the number of your local LDA (Almost every state has one). They can give you information about learning disabilities and your legal rights, and may also be able to refer you to a psychologist.
You can contact a school psychologist who has a private practice, a clinical psychologist, or a clinical neuropsychologist to assist you -- just make sure they have expertise in LD and school-related problems. You can also get a referral from your daughter's pediatrician. Most large hospitals have learning disabilities clinics that can provide the service you seek. Professionals can also help to determine whether your daughter's difficulties are related to the divorce, or some other issue.
If the testing you have done confirms that your daughter has a learning disability and your ex-wife doesn't agree, you can ask the person who did the evaluation to explain the results to her and try to help her see the importance of intervention. Hopefully, you will both be able to set aside your differences and disagreements long enough to get your daughter the help she needs.