How do you find out what is the concentration of the solution?

two methods
(1) by titrimetric method: in this method, concentration of the solution is determined quantitatively by react or titrating equivalent amount of compound (which is to be determined and have unknown concentration) near another compound of known concentration and particular volume.
N1 . V1 = N2. V2
N1 = (N2 . V2)/V1
N1 = normality of compound of unknown concentration.
V1 = volume of compound of unknown concentration consumed during titration or reaction.
N2 = normality of compound of particular concentration.
V2 = volume of compound of known concentration consumed during titration or spontaneous effect.
(2) spectrometric method:
in this method two solution is prepared: first is the tryout solution which contain compound of unknown concentration and second is the standard solution which contains same compound of known concentration. graph is obtain and by comparing graph and their extinction coffecient, concentration of compound of unknown concentration can be determined.
for detail see A. I. Vogel's textbook of quantitative chemical analysis.
2 ways
1) Assuming it is visible by some from of spectrometry, net several known concs of equal soln. Draw a graph (stnd curve ) plooting these points. Measure the unknown, and read on the graph what concentration your spectrophotometric reading relates to.
2) send it down a mass spec and determine how much is surrounded by your original example (quicker and easier IF you have a mass spec and you know how to use it!)
titration next to a known solution im guessing