In general, there are two schools of thought among therapists.  Some will not speak to others with regard to shidduchim: they fear that this may compromise the therapy and prevent the client from really sharing their deepest secrets out of concern that the therapist may disclose it. Most therapists, however, are willing to speak with others if this is the client's preference. To the best of their ability, they will disclose the reasons that the client began therapy, what they accomplished, and how this may (or may not) affect them in marriage. 

 

Please realize that everyone has struggles and challenges and stress and worries. It is just part of the human condition. Those who go to therapy do so to work on themselves and strive to become the best they can be during our short stay down here. This is something positive and commendable and is a plus. At the same time, you are correct for looking into this. Marriage is a very serious commitment and you want to do research to minimize surprises that can end up in broken engagements, divorce, or miserable marriages. 

 

When you speak to the therapist, be as honest as you can and listen to what the therapist is saying. You should inquire what issues brought them to therapy, what they worked on in a general sense, and, most important, how this can affect them in marriage. As mentioned previously, most people enter therapy for run of the mill issues that all of us experience such as stress, anxiety, family issues, etc. The main point that you should look out for is if they were resolved in a manner that will not significantly impact a marriage. From a hilchos loshon hora standpoint, it is probably wise to ask specific questions. You may want to ask if there are significant  challenges that you should be aware of such as serious mental illness, personality disorders (such as narcissistic or borderline), serious addictions, or LGBT related concerns. It is probably worthwhile to also ask if the person is taking medication, not because that will negatively impact a marriage. In fact, most psychiatric medications are common and mild, but some (depending on the type of medication) can indicate more serious conditions that you should at least be aware of. 

 

I'd just like to conclude with two points that are important to keep in mind. First, those who seek therapy are generally the type who are humble and growth oriented and these are positive qualities in marriage. Second, as mentioned earlier, it is important to realize that Hashem made us with all struggles--no one is perfect. Those who are honest with their imperfections up front are praiseworthy. Hopefully, we can recognize the limitations of ourselves and of others, and, at the same time, look at the big picture and be able to see the positive aspects and the wonderful human being. May you have much hatzlacha in navigating this.