1. Choose the location (Gartner reveals TOP 30 locations for offshore software development services, including Belarus (Eastern Europe)).
2. Find reviews about this services at Clutch https://clutch.co/developers/belarus
3. Most vendors will have a portfolio you can check out to see examples of their past work. https://belitsoft.com/offshore-software-development-company/portfolio
4. Your vendors should also have a list of satisfied past clients that can vouch for the quality and dependability of their service.
So, you can take a look at the offshore software development company Belitsoft from Minsk (Belarus) https://belitsoft.com