If you want to find a reliable vendor for cloud migration to AWS, pay attention to the following criteria: technical skills, years on the market, clients and portfolio, feedback from customers, communication style, and willingness to deliver value.
Of course, the first source of information is Google where you can search for companies manually based on the results available. However, you may start by asking for recommendations if you know who to ask. This seems to be a more reliable way but you will still end up checking all recommended companies via Google.
One more efficient way is to use special listing websites like Clutch which categorize all listed companies by services provided and give general information about them, provide references from clients, and showcase their portfolios.
Once you shortlist 4-5 providers of migration to AWS services, prepare for direct communication with them. Make a list of questions that will help you to choose the right vendor. Ask them about their process, how they provide data security, how they communicate and report on tasks, and ask about the expertise of tech specialists.
You can also make a table with such points as rates, project estimate, timeline, relevant expertise and capabilities, references, level of involvement, etc. It will be helpful to compare the information you collect and choose the right AWS partner for your migration to AWS.