Comment # 10 on bug 89059 from Sebastian Parborg

(In reply to Jarkko K from comment #9)
> How can I swap between those 2? 
> 
> I don't know what linux mint 17.1 uses as default.

You can see it in your Xorg log.
If it manages to load "radeon_drv.so" and/or "ati_drv.so", and them prints:

(II) RADEON: Driver for ATI Radeon chipsets:
        ATI Radeon Mobility X600 (M24) 3150 (PCIE), ATI FireMV 2400 (PCI),

With a long list of cards, then you are using the video-ati modesetting driver.