In the MobileAction platform, open Top Advertisers and filter by your category, country, and platform. The apps investing most heavily in paid UA line up with their ad networks, creative counts, and spend behavior attached. From there, click into any advertiser's profile to see which networks carry their ads and how many unique creatives they rotate, and use Top Creatives alongside it to see the actual ads earning those impressions. Checking the list monthly shows you when a new player starts spending, which is usually worth knowing before their ads show up next to yours.