If you have access to the purified enzyme, there are a variety of assay kits available, such as these:

http://www.abcam.com/acetylcholinesterase-assay-kit-colorimetric-ab138871.html

http://www.sigmaaldrich.com/catalog/product/sigma/mak119?lang=en®ion=US

Prepare a set of serial dilutions of your extract, mix it with the enzyme and substrate, and measure the percent inhibition compared with a control. Graph % inhibition v. extract concentration, and fit the curve to a suitable equation by nonlinear regression to measure IC50, the concentration of extract that inhibits by 50%.

Make sure you check for interference in the assay by the extract. For example, if the extract is colored and you are using an absorbance assay, you will have to subtract the absorbance of the compound before calculating % inhibition. You can measure this interference separately.

thank you for this information

Please find the method in attached paper