How do to determine if an adapter is needed for disc brakes and rotors?
Just to enrich this answer, there are the two systems nowadays for mounting the brakes on the fork and frame; these are IS or postmount:
http://www.pricepoint.com/TechPages/TECH_Page_Disk_Mount_Types.htm

Normally you can find adapters to use one or the other.

Regarding the disk-hub mounting, there are also two options, 
- 6-bolt (the name is self-explainable, it is the standard in the market):
- or centerlock (Shimano proprietary), which are fixed by a lock ring, similar to the one used to fix the cogset to the rear hub, but on the disk side:
http://www.pricepoint.com/TechPages/TECH_Page_Rotor_Types.htm

You have adapters to mount 6-bolt rotors into centerlock hubs, but the opposite is not possible.

Just one final consideration, if you are running a 26" wheel, current brakes allow a huge braking power with relatively small rotor. Nearly nobody would use more than 160mm on 26", unless you are doing freeride or DH.

Regards.