Search for company name
Searching for their company name inevitably leads to listings of directories, articles, and social media pages. However, if the company is relatively large, there will be some feedback somewhere.
It's very likely that someone who has had good or bad experiences with SEO companies will mention it on a webmaster forum or blog.
It is very important not to consider these nominal rankings as this is the way some unethical SEO agencies advertise their services. If you are looking for more information about Dallas seo agency check this out.
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Check out the portfolio
If an SEO agency is good at what it is, you will get endless recommendations for that. Don't be afraid to ask them for examples of their previous work. If the SEO company serves a large number of clients but displays few references, ask why, or better, ask the website owner. Webmasters are more likely to tell you what isn't working or not than what is working well.
Check your connection
Most online marketing is backlinks and there are plenty of free tools for checking links to websites. The quality of the websites that link to this SEO company reflects the ethical standards of their work.
If you find that an advertising agency has a ton of backlinks from websites that you perceive as spam, then this is most likely the approach they are taking to market your website, which goes against all basic search engine guidelines.