Effort estimation of change request (CR) for an app built on react native

We are looking for a consultant who can help us with effort estimation for changes required in our app. As the app was not developed in-house we rely on a third party for the development and changes that are made to the app. Being from a non-technical background we find it difficult to verify and bench mark the cost quoted to us for any CR (change request) that is made to the app. The app is built on React Native and available on iOS and Android. We are looking at a developer/consultant who has an expertise about the language and help us validate the estimates shared by our third part developer. If you are an expert do ping me. I will let you know more about the app.