What is the best approach with regards to designing a website for somebody who's business does not seem legitimate.

Most cases with small companies i do not worry , but recently i had a 'engineering' company. of course i asked them for any certifications and registrations as from the little i know these things mean a lot in the engineering world.

They could however not provide me with any.

Do i as a web designer and content writer have any responsibility regarding the validity od this company ?