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Could look at their source code. Fancy schema markup is pretty common from an SEO agency (arguable how beneficial it is).
If you have access to ahrefs/similar can see if they have a lot of suspicious back links to their articles. An inflated DR score is a sign theyve been buying backlinks. Same with their GMB reviews. The cadence/volume of their blogs is probably indicative of involvement. Finally try doing a search of your competitors logo, might get lucky and find it on an SEOs we site advertising they do work for them.
