I use the chrome extension and when im on the site i can open a drop down menu with everything running on the site. I look for tag managers, ad exchanges, retargeting tags, data reseller tags, ad tech tags, etc... With one glance i can usually tell how sophisticated of a media buyer they are and if they seem eager to experiment with new ad offerings. (i.e. lots of small and obscure ad services or expensive data+programmatic exchanges like chango, etc...)