Seen a few black widows around my house after searching their nest/web area for them.
You can tell it's a black widow before you even see it because if its web. They have a unique style of web making and you can't misplace it for any type of (common) web out there. The web is a mess, it looks like its "not taken care of", no upkeep, it's just a mess of crap and web.
Most webs are clean and normally have a nice place to set up, and many have designs.
This is an example of black widow nest:
If you just look at some pics of their webs, you'll always know if there's a widow around.