There are 4 primary components to choosing the correct keywords. If you know what they are then you will be able to better screen an seo consultant that you sc seo consultant are thinking of hiring.
The keywords chosen must be relevant to the content that is on the site being optimized. When someone does a search for a particular keyword they want to find content exactly matched to that keyword. The search engines want the searcher to find this content as well. Either the keywords being chosen for your seo campaign need to match what is already on your website or the seo consultant will need to create content that will match the keywords chosen for their relevancy to what services and products you provide.
The keywords used to optimize your site need to be getting a decent amount of traffic. This might sound redundant and completely obvious, however I have heard of many an seo consultant choosing keywords that are getting zero to little traffic because they are easier to rank for. This does not include the practice of going after "long-tail" keywords. Long tail keywords get less traffic than major 1-2 word phrase keywords but they do get enough traffic to warrant going after. In general a keyword should be getting at least 100 searches per month.
Keywords that are relevant and that are getting decent traffic should also be chosen for the absence of a huge amount of competition. This will shorten the amount of time it takes to ranked and will lessen the overall cost of the campaign. A good seo consultant will take this into consideration.
4. Commercial intent
Not only should keywords be relevant, get decent traffic and not have a ton of competition but they also need to have commercial intent. This means that the keyword looks like a keyword someone would use when searching to buy something. For example the keyword "seo" might be searched mostly by people who are learning about seo. The keyword "seo consultant" or "seo services" looks more like the type of keyword that someone looking to hire an seo provider would use - this is what is meant by commercial intent and your seo consultant needs to be aware of this. Visit http://serpalliance.com/sc-seo/ for the best SEO consultant services.