When you start building your online business and are looking for a web design company that will cater to all your website needs, you need to understand that this is a very serious step because you trust your company and its reputation and future.
1) Availability of portfolio
Yes, it seems obvious that a web design company should have its own website with a portfolio of previous work.
2) Total number of jobs
It can tell you a lot, but there are no standard numbers that guarantee you're dealing with a web design expert. Yes, 50 successfully completed projects are better than 5, but quality matters too. Compare that number to the length of time the company has been in the market and calculate the average number of projects for a year, month, or week.
3) Type of work
If you need a particular technology to implement your website, it is more than necessary to look at examples of how web design companies work with it.
However, sometimes it happens that if the technology is not used often and is very popular, web design companies cannot work with it for commercial purposes, but can be used to develop internal projects so that you do not lose anything.