DCP-o-matic includes a DCP player, and although it requires a very high-speed CPU to play DCPs in full resolution, it can also play DCPs at reduced resolutions with slower CPUs.
To use the player, start DCP-o-matic Player, and load a DCP using the Open option on the File menu.
If you load a VF and/or encrypted DCP you can add your OV and/or KDM using the appropriate options on the File menu.
During playback the Performance area at the bottom right of the window will give details of how many frames are being dropped; these are frames that were not decoded from the DCP quickly enough. If this number is high you can increase performance at the cost of rendering quality by choosing an option from the View menu. If you set the player to decode at less than full resolution the DCP's data will be decoded at this lower resolution, which is quicker than decoding at full resolution.