This is a very common question and one that we often get asked.
Here is a link to an article I wrote recently https://homenergymk.org/how-to-find-a-builder-you-can-trust/
What I would say is be inquisitive, do some digging and find out as much about the company as you can!
Check companies house at the very least and make sure you can speak to previous customers and ask to see the work. The key questions are : did they finish the job? To the standard you wanted? and would you have them back for further works?
Navigating the landscape of rogue traders can be tricky but trust your instincts, if it sounds too good to be true it probably is!
One more word of advice; always have a contract before any works commence and there is no excuse to not offer one
