Attached article, Standardization of the fourier transform near-infrared reflectance spectroscopy for estimation of some oil quality parameters in mustard (Brassica spp.) by J. Singh, P.C. Sharma, S.K. Sharma, A. Kumar; Plant Soil Environ; Vol. 59, 2013, No. 10: 478–483

may help you

NMR can do that analysis using low field NMR instrumentation. There are many descriptions of this analysis on the web. Seach on "oil content by NMR" in google and there are many examples and papers. One good example with mustard oil content described is:

http://www.new1.dli.ernet.in/data1/upload/insa/INSA_1/20005ab6_731.pdf

If you wish to assess few samples with low cost, use Soxhlet extraction method.  The oil extracted is expressed as per cent of weight of seed.

Pl see http://www.academia.edu/2967956/FAT_and_OIL_FROM_OIL-SEEDS_PART_I_SOXHLET_EXTRACTION

If you are able to measure the oil density (g/cm3) and accuratly determine the whole volume you got from a given mass of seeds than you may determine the yield in %

Mushtaq and John have already provided you with some very sound advice, but just to add to the potential measurement techniques you might also consider NIR. This is a very high throughput technique requiring only seconds per sample and can be used for small sample sizes. I am attaching a study that demonstrates this technique with 13 mg sample size.  A bonus is that the seeds do not need to be ground or destroyed which is very useful for breeding programs.

http://www.asdi.com/getmedia/3ba25479-a28d-4355-9a26-1c81cc453db9/Small_Sample_Size_Evaluation_using_LabSpec_4_Standard-Res_i_web.pdf

Thanks for providing the necessary information regarding the query. Highly obliged for this.

Soxhlet method is simple and can be done in ant chemistry lab.NMR method is quick and accurate.It depends on facility you have. Consult the website of Directorate of oilseeds research,Hyderabad.