Drug solubility can be find out using drug release studies.The attached article can give you an idea about drug release..For drug release you have to choose suitable membrane and solvents according to the drug...

you can perform solubility test similar to saturated solubility, and determine the soluble drug in lipid by sampling and diluting with organic solvent and using suitable analytical instrument depending on the sensitivity

you can utilize "shake flask method" to determine solubility of your subject drug with targeted lipids. methodology for shake flask is available in many research reports. 

Refer this article:

Computational Prediction of Drug Solubility in Lipid Based Formulation Excipients

Linda C. Persson, Christopher J. H. Porter, William N. Charman, and Christel A. S. Bergström

ABSTRACT

Purpose

To investigate if drug solubility in pharmaceutical excipients used in lipid based formulations (LBFs) can be predicted from physicochemical properties.

Methods

Solubility was measured for 30 structurally diverse drug molecules in soybean oil (SBO, long-chain triglyceride; TGLC), Captex355 (medium-chain triglyceride; TGMC), polysorbate 80 (PS80; surfactant) and PEG400 co-solvent and used as responses during PLS model development. Melting point and calculated molecular descriptors were used as variables and the PLS models were validated with test sets and permutation tests.

Results

Solvation capacity of SBO and Captex355 was equal on a mol per mol scale (R2 = 0.98). A strong correlation was also found between PS80 and PEG400 (R2 = 0.85), identifying the significant contribution of the ethoxylation for the solvation capacity of PS80. In silico models based on calculated descriptors were successfully developed for drug solubility in SBO (R2 = 0.81, Q2 = 0.76) and Captex355 (R2 = 0.84, Q2 = 0.80). However, solubility in PS80 and PEG400 were not possible to quantitatively predict from molecular structure.

Conclusion

Solubility measured in one excipient can be used to predict solubility in another, herein exemplified with TGMCversus TGLC, and PS80 versus PEG400. We also show, for the first time, that solubility in TGMC and TGLC can be predicted from rapidly calculated molecular descriptors.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841656/

In your lipid of choice, add excess drug, vortex/mix very well. Take sample of the solution, dilute it in some organic solvent which will solubilize both drug and lipid. Determine the concentration using HPLC with the help of a std curve. 

Solubility of drug in lipid matrix can be determined by weighing excess lipid (~100 mg), varying amount of drug (~1-50 mg) and dissolving it in common organic solvent (~10 mL). The solution can be casted on a petridish to obtain a thin lipid film and allow to dry at room temperature. Then observe under light microscopy for drug crystallization. The lipid film with particular drug concentration shows crystals indicate its maximum solubility.

Thanks to all for quite useful suggestions