If you are an experienced professional or should you have experience in a particular field, you then don't desire to register with any old recruitment agency, you would like to find one which will develop the type of positions that you would be interested in, if not the only jobs you might be offered may simply be inappropriate for your skills and experience.
There are a huge selection of specialist agencies out there, most of which can be found on the internet. So if you are a marketing guru, someone who is skilled in one of the many facets of construction, or you are a chef etc then don't get a recruitment agency that does 'everything' you will need one that specialises in your field.
Basically homing in your search when on the web, should help you to locate the right kind of agency. If https://inspireselection.com/ in recruitment agency, you will have literally a large number of options to search through, but in the event that you type in 'Construction Recruitment Agency' then this will start to narrow the options down.
Basically though, a number of the search engine results will have 'keywords' mounted on them, which means that if the Webmaster for a niche site tags the website to be flagged up if 'construction' is typed in, then whether or not or no construction jobs are on the website, the website will be high up in the search engine, therefore the more specific you could be when searching, then the better it is, so if you want a construction agency that could offer you a job as a Quantity Surveyor or someone who draws plans using CAD, then type in Quantity Surveyor or CAD designer as well recruitment agency and this will soon start to narrow down the search engine. So the more specific you may be, the better!