There are so many SEO agencies around and from experience I will discuss some of the tricks they play.
Starting the deal:
At the start of the SEO deal you need to be very careful what you sign up for. An SEO sales person will normally do the sale by signing SEO contract with you. You should be aware of terms of services and responsibilities assigned. e.g. who will develop the new web pages, who will write the content of pages, who will optimize the web pages and how many pages will be optimised, will you give the SEO Company the access to the web server? Payment details, keywords and so on.
#1 ranking guarantee:
Many SEO companies will trick you by advising you “we will guarantee you #1 position on Google”. This guarantee is the most misleading of all. If any SEO agency guarantees you #1 position on Google then you should instantly ask them how much control you have over Google algorithm. They will simply know you are not the person to be misled because you have done your home work. The search engine always changes algorithm and at any time the best SEO Company in the country will be caught by this change and your ranking will be instantly affected. This means it may drop or certain occasion may climb. Google advises the business community not to trust an SEO company that guarantee you #1 position as this is basically mean the SEO company know someone inside Google who gives away Google secrets. A legitimate Guarantee will be such as increase the number of visitors. Or increase in the enquiries.