Too less info for answer. NOSY of substituents should be of help. Also one may try to simulate UV-spectrum for both isomers and if they are significantly different - compare them with experimental. One another option would be chemical transformation and X-ray of product if it will be crystal. Might (not obligatory) include transformation of double bond to smth different: epoxidation or diels-alder reaction.

Dmitry Sharapa Thank you for your helpful answer. Unfortunately, NOSY of substituents is not effective for determination and UV-spectrum isn't either. Proper chemical transformation may be of a great help. Thanks for your answer. I will inform you if the determination is successful. Thank you so much

Good suggestions by Dmitry; may add few things as well:

1) is the phosphorus in P=O group in P(III) form ? If yes, it could be oxidized to P(V) which might be crystalline;

2) try to make other similar derivatives, perhaps some of them could be crystalline;

3) can your substance or its derivative (like an oxidated form, for example) make a salt with acid or base, crystalline in nature ?

4) making the opposite isomer could be helpful (it can be crystalline, or show some other properties helpful in configuration assignment);

5) how about 13C and 31P NMR, especially if you have a series of similar compounds ? You may see some clues in chemical shifts or splitting, useful in determination of configuration.

As Dmitry said, more information about the structure will be helpful in giving better advice.