Although more and more keywords are getting encrypted by Google every day, another smart way to come up with keyword ideas is to figure out which keywords your website is already getting found for. To do this, you'll need website analytics software like Google Analytics or HubSpot's Sources report, available in the Traffic Analytics tool. Drill down into your website's traffic sources, and sift through your organic search traffic bucket to identify the keywords people are using to arrive at your site.
QUOTE: Each piece of duplication in your on-page SEO strategy is ***at best*** wasted opportunity. Worse yet, if you are aggressive with aligning your on page heading, your page title, and your internal + external link anchor text the page becomes more likely to get filtered out of the search results (which is quite common in some aggressive spaces). Aaron Wall, 2009
Ask for a technical and search audit for your site to learn what they think needs to be done, why, and what the expected outcome should be. You'll probably have to pay for this. You will probably have to give them read-only access to your site on Search Console. (At this stage, don't grant them write access.) Your prospective SEO should be able to give you realistic estimates of improvement, and an estimate of the work involved. If they guarantee you that their changes will give you first place in search results, find someone else.