I recently started a new project and needed a domain name and matching social media channels.  This particular project revolves around images so Instagram was the most critical social stream.  My top pick for a domain name (bostonkeyword.com) was available, fantastic. I went to Instagram to check my name and a tween was using the name for his collection of 5 Bruins game photos. I could try to buy his twitter handle but this was a low budget operation and tweens are probably wily social media negotiators. Back to the drawing board, but this time I used a tool to do the checking for me, this is one of the many I found:

namecheckr.com – Social and Domain Name Availability Search For Brand Professionals

name-checker
It seems very accurate but I would use it as a first pass and then go manually check the actual social channels that are critical to your success. It’s also a good idea to make sure you haven’t inadvertently selected a name or hashtag that has negative content associated with it.  Search for both @theNewName  and #theNewName on all channels.