It wouldn't be easy, quick, or super accruate, but you could create a bunch of different facebook profiles, and then use each one to do searches for certain things on facebook, certain companies profiles, etc. Make each profile a different age, different sex, different income (it guesses income based on education level and job title). Then see which ones see ads, and which don't.

similarweb.com is another site that will show you where competitors' traffic is coming from...if they are large enough. It can be quite educational, especially paid vs organic traffic info.

http://adfinder.co/?aid=271965

AdFinder is a tool I use pretty regularly (along with a few others) to find all keyword (branded or otherwise) relevant posts and ads on Facebook.

Another similar hack is to use GrowthBot from HubSpot (installed on facebook messenger or Slack) and ask GB what keywords a competing company is buying in adwords.

I hope this helps!

Facebook recently made it super transparent. You just need to go to your competitors' page and look at the tab on the left. You will see "Info & Ads". Inside you'll see the active ads, the one that are running right now.