Are Macs vulnerable to the recent SSD encryption vulnerability?
This does not affect Mac because Apple doesn't use the SSD's built in hardware encryption algorithm. Apple's FileVault uses  an XTS-AES 128 cipher that's  "tied" to a password and protects the boot partition of your drive (the EFI is not).

The SSDs mentioned in the article can have the master password "reset" by simply writing a single bit to the SSD's firmware.

Operating Systems like macOS and FreeBSD boot an un-encrypted partition first, then it boots the encrypted one based on a password/phrase/credential supplied by the user. 


  Does MacOS trust the hardware encryption of the SSD drives, or does it always use software encryption?


It doesn't trust it nor distrust it.  It uses it's own encryption rather than what's included with the drive.


  Is there a way to check whether SSD-based or software encryption is being used?


For macOS through Sierra (10.12.x):

diskutil info diskX | grep -i encrypted


For macOS High Sierra and (10.13.x):

diskutil apfs list | grep -B6 FileVault