The Device.QueryCapability method should give you the information you need.  However, that depends on the driver reporting correct results.

The real way to get this information in TWAIN is to use the DG_CONTROL / DAT_CAPABILITY / MSG_QUERYSUPPORT triplet.  The driver will return a bit field containing the supported triplet information.

[minutes later...] 
Ok... I took a little time to write a demo which will use the TwainController methods to get this information.  I've attached the (C#) project.