Ask the piano accompanist for a repertoire list, videos of their playing or whether they are familiar with the repertoire you will be performing. Some accompanists lack sensitivity in their playing. Accompanying is different from solo piano playing in that it involves the collaboration of two or more individuals. Watching a video of their playing will allow you to get a sense of their piano playing and perhaps accompanying style.
There are a number of pieces that are challenging for a collaborative pianist. I am skilled in playing Negro Spirituals (these are the songs of my ancestors and reside within my soul), Italian, French and German art songs as well as an extensive list of instrumental collaborative repertoire. I have performed all pieces on my repertoire lists. I can accompany middle, high school and college instrumentalists (all instruments) as well as vocalists.
Also, ensure the accompanist actually possesses a Masters degree or Doctorate of Musical Arts (DMA) in piano or collaborative piano. A Master's or DMA signifies an individual has met certain competencies in piano. It is also important for your accompanist to be skilled in some solo piano repertoire AND collaborative piano repertoire. Certain composers require different types of piano technique. I have done both.
A good accompanist also possesses excellent sight-reading skills, transposition and aural skills. Accompanying is not just about learning notes on a page but being a collaborator and supporting the soloist. The soloist and collaborative pianist form the ultimate partnership. You want to choose a pianist who is sensitive to your style of playing.
I possess two master's degrees. My first Master's degree was in Piano Pedagogy and Performance from George Mason University. As a Master's piano student, I was required to present a solo piano recital, present a lecture-recital (I presented one on Johannes Brahms), take a vocal accompaniment class (transposed Italian art songs on sight), an instrumental accompaniment course and complete two semesters of chamber music.
The second Master's degree was in Injury Preventive Keyboard Technique from Salem College where I studied with the renowned Dr. Barbara Lister-Sink. Again, I presented a 50-minute solo piano recital and a mini lecture recital on William Grant Still.
I also trained in the Russian piano method and am knowledgeable about stylistic nuances for various composers. I know how to adapt my sound to blend with the soloist. I have been working with my former teacher from the Tchaikovsky Moscow State Conservatory over Zoom and it has enhanced my collaborative and solo playing.