My thought process: they could gather about 100 of these specific ICPs they are making an assumption of.
From there you can enrich their data by looking at the following factors: their industry, company size, location, tech stack, hiring signals, revenue, growth stage, funding, and website traffic. Tools like Similarweb, Semrush, BuiltWith, Apollo, and Crunchbase can help with this.
After that, you could segment these companies by industry, employee size, job titles, or growth signals, depending on which data you feel would be most needed for your client.
Using the data you have gotten, you can run an experimental campaign with different ICP variations relating to what your client is offerings. This will help you gather data on which profiles are responding well to your message.
To be extra sure, you can also use AI to analyze the data and ask questions.