You need to titrate the NaOH against a known strengh of stong acid such as HCl (hydrochloric). If the NaOH has absorbed water / CO2 then it will come out waker than you would expect from pure NaOH.
Use pure ( or De-ionised) water for making up the solutions, not tap water or rain water as they will likely skew your results.
I would try using 4g of your NaOH in 1L of water (these should both be accurate measured) and titrate against 0.1M HCl solution - if you can get some. If you cannot get any known strength HCL solution, then you could try 4g/L of fresh NaOH and use that as a reference. Titrate both solutions against an scid solution sucha s HCl of about 0.1M. Brick acid is around 35% HCL iirc, so diluting so try 10ml of brick acid HCL in 1L of water as a starting point to get an HCL solution in the right range. Other acids could be used instead, such as sulphuric, or even white vinegar.
White vinegar is normally between 4 an 7% but some "cleaing vinegar" can be 20%. You will need to dilute the vinegar make 100ml upto 1L for titration puposes.
Because NaOH will absorb both H20 and CO2, the titrations should indicate the amount of Na present from which you can calculate the purity. Simply divide the titration value of your old caustic by that you get for the fresh caustic an multiply the result by 100 to get the %purity.
