Ask the company you're thinking of hiring about the techniques they employ. If you doubt the quality of their practices then it's best to move on and search a company that best fulfils your goal by playing by the rules.
There are companies with impeccable track record in SEO and SEO services and there are companies who base their SEO services and activity on breaking the rules. Asking for references from a SEO companies clients may not always make clear whether the company is ethical or not.
Also clients in a competitive industry may want to tie their SEO Company into exclusive deals or not want to proactively let them get more clients that would be their direct competitors and their Websites
Ask for explanations if something is unclear. If an SEO creates deceptive or misleading content on your behalf, such as doorway pages or "throwaway" domains, your site could be removed entirely from Google's index. Ultimately, you are responsible for the actions of any companies you hire, so it's best to be sure you know exactly how they intend to "help" you.
Also do the SEO company do all of their work in your country do they outsource this to cheap labour. This has become common and in most cases the quality of work suffers to cut costs. Therefore Cheap SEO is not Good SEO.
