You can google this and select a few websites that are organically ranking high in SERPs. The bottom line is to check what they have done for their brand in terms of digital marketing and how are they growing as a team.
Sometimes, the youngest digital marketing company in sri lanka might be the hottest chilli in the block and the easiest to work with because the smaller the team the higher the focus on your product/brand.
Also, if quality work is expected as a client undervaluing work of an agency must be avoided, and great work comes at a premium price tag.
If the digital marketing company in sri lanka that fits you has an advertising background you might get creative content. However, in terms of execution, this might not bring you much benefit since algorithms understand simplicity more often than creative content.
Hence, you must find the balance when it comes to understanding which company is best for digital marketing.
Make certain that the top end of the company are trailblazers. So that they are best at what they talk about.
So in conclusion, you need to first check for yourself who does it best on Search Engines and then check their background. Also, have an open mind about the cost and always have a fixed budget for negotiating the deliverables.