The same way it's tough to buy a car knowing nothing by what it ought to look like underneath the hood, it's a bit intimidating to pay for a search engine optimization company if you aren't exactly sure what distinguishes the great in the bad. Since internet search engine algorithms are extremely temperamental, you need to make sure you are hiring a company that is experienced and up-to-date; otherwise, all the services you taken care of may go the window as soon as the following Google update is rolled out. Here are some tell-tale signs to take into consideration - if your company does these, you might want to forgo hiring them.
In case your company promises an unreasonably many articles, blog posts, links, social bookmarks, et cetera monthly, you should definitely back off. Google wants genuine, organic content - not something that has been spewed out by a program in broken English and questionable grammar. These 500 articles and 1,800 social shares might look appealing, but when they are not written by a human, they are able to do more damage than good.
No references or examples:
Prior to hiring a company, ask to determine the things they were able to do for other clients. They should have some sort of track record, a way of showing you their work has been successful previously. Worthwhile SEO company keeps visual records and statistics of progress with time to show for their clients. If they don't leave a paper trail of any sort, run away!
They are not well-rounded:
The company should push not only high rankings; they should be also promising more sales, more site visits, more Twitter followers, etc. Sure, getting your page show up on the front page of Google's results is awesome, but when no one is hitting the hyperlink simply because they will easily notice it isn't useful or relevant, it doesn't really help you any. The best goal would be to produce a better representation of your brand and to form a good reputation with people to the website.