Nuke provides real-time scopes to help you evaluate your footage. Real-time scopes provide data as the Viewer cache plays back, but you can also view the scopes frame by frame if you prefer. Scopes are accessed from the contents menu, Scope sub-menu.
See Histogram, Waveform, and Vectorscopes for more information.
NOTE: Viewing scopes may slow down playback. If this is the case, make sure Include viewer color transforms is enabled on the Scopes tab of the Preferences dialog. If the playback frame rate is still low, click the optimize Viewer during playback button 
in the Viewer to stop updates elsewhere on the interface during RAM cache playback.