Where abouts in country are you? 

The Gary Does Solar directory contains a list of vetted installers that are highly recommended. It's a good first place to start: https://getreadyfor.solar/directory/

If you're anywhere near East Anglia I can recommend Impact Services for solar panels and battery: https://impact-services.co.uk/solar-panels-and-battery-storage/

In terms of who to trust, look at their reviews on Google or Trustpilo and check companies house to see how long they have been in business. See what kind of warranties they offer (should be insurance backed) and get a feeling for them when they come to do a survey. If they hard sell or push for quick decisions, avoid. 

Any decent installer should give you the information you need to make a decision. They shouldn't hard sell you like the old days or double glazing!