My first stop is
builtwith.com to see if they have things like : CRM, analytics, pixels for remarketing (all big things)
Then I use an seo tool. Are they ranking high for non-branded terms (aka are they good at seo). Using backlink explorer, I can also see where their PR is coming from. Are they getting press from outlets (these will be high domain backlinks). You can also check paid search and display.
Those would be where k would start but next you’d have to look into their GTM a bit more around PLG, Sales, etc.