There are many methods for evaluating the frequency response of a room but one of the simplest is to "stimulate" the room at different frequencies, record the result and evaluate the returned wave.
A simple test track might look like this:
A series of sine waves at different frequencies are recorded and normalized (brought to maximum volume) in a test file at the following frequencies.
(What the actual frequencies are is relatively arbitrary but the relationship between then should be about 1/3 octave or less).
For very fine resolution you can record chromatically but the file will take a long time to play
The leading edge of each burst of sine waves is faded in slightly so that the stimulation of the room is gentle rather than sudden.
This helps prevent flutter. Play the track into the room and record it back into a recording device then examine the result which may look something like this
Normalize the track to bring the recorded wave to maximum volume and the tallest returns will be your standing waves, in this case 100hz (4th wave from the left).
Most sound design software will have some facility for showing the level across a section of audio. In the case of the program Peak you can select a wave and choose Find Peak from the DSP menu.
This tells me that the wave at 15 seconds is -2.5 db lower than my standing wave.
By noting all of these we can build a map of volume against frequency.
Using a simple spreadsheet we can enter these values and even chart the result giving us a response curve for the room.
The dips at each end are almost certainly the result of limitations of our speakers/mics and basically should be ignored.
However it does point to an important weakness in the method - our test equipment.
Neither speakers nor mics are perfect but by going to the manufacturers website we can usually find a frequency response curve.
These curves can be factored into this map to give a better idea of the system.
Remember you cannot just add decibels, use the calculator Adding Decibels further dorn this page.
With this information two options are available.