I have 3 loadbearing (2x6) walls in my basement - one of which is against the foundation and holds half of my house up, while the foundation supports a cantilevered beam for a bump out.

Every single other wall in my entire house (except these load bearing walls in the basement) are 2x4's.

I'm just saying that most of the time, when you come across a 2x6 wall - it's load bearing. Since OP can't tell either way, it's better to be safe than sorry.