Any reputable company will file accounts in the public domain and a simple way is to check at Companies House or any credit rating agency.
Look for wholly independent schemes that hold UKAS accreditation and not ones run by supplier memberships
Check the wording, "compliance to" or "tested in accordance with" does not mean products have passed those tests. Test certificates should be sought ensuring they are UKAS registered bodies.
To be certain your windows do not come from illegal logging always check for either FSC of PEFC certifications. These are an independent chain of custody certifications which ensure products come from well managed forests.
Look for certification from independent accreditation bodies such as BM Trada.
Buying the wrong style or poor quality products can have a detrimental effect on the look of your property. Well purchased timber windows can enhance any property and add considerable value to your investment.
This should detail exactly what you are buying, what you will pay and the terms of payment. If you are not sure then don't be afraid to ask.
The more knowledge you gain, the more precise you can be regarding your requirements, this way there is less chance of misinterpretation.
Ensure the company has UK based operations and field personnel who can help resolve issues.
Joinery needs to be maintained, ensure you receive homeowner manuals and warranty information.