How to find out which loader is causing this warning: 'loaderUtils.parseQuery() received a non-string value which can be problematic'

I have recently upgrade webpack to v2.x. One of the loader is giving out this warning:

    Trace: loaderUtils.parseQuery() received a non-string value which can be 
problematic, see https://github.com/webpack/loader-utils/issues/56
    parseQuery() will be replaced with getOptions() in the next major version of
 loader-utils.

I have updated all the loader to the latest version but this warning remains. How can I identify the offending loader?

Solution

Add this line to the webpack.config.js

process.traceDeprecation = true

it will print out the stacktrace

Trace: loaderUtils.parseQuery() received a non-string value which can be problematic, see https://github.com/webpack/loader-utils/issues/56
parseQuery() will be replaced with getOptions() in the next major version of loader-utils.
    at Object.exports._printDeprecationMessage (internal/util.js:30:13)
    at deprecated (internal/util.js:54:22)
    at Object.parseQuery (/Users/antkong/dev/project/node_modules/loader-utils/index.js:78:3)
    at Object.module.exports (/Users/antkong/dev/project/node_modules/postcss-loader/index.js:46:30)
    at LOADER_EXECUTION (/Users/antkong/dev/project/node_modules/loader-runner/lib/LoaderRunner.js:119:14)
    at runSyncOrAsync (/Users/antkong/dev/project/node_modules/loader-runner/lib/LoaderRunner.js:120:4)
    at iterateNormalLoaders (/Users/antkong/dev/project/node_modules/loader-runner/lib/LoaderRunner.js:229:2)
    at iterateNormalLoaders (/Users/antkong/dev/project/node_modules/loader-runner/lib/LoaderRunner.js:218:10)
    at /Users/antkong/dev/project/node_modules/loader-runner/lib/LoaderRunner.js:233:3
    at context.callback (/Users/antkong/dev/project/node_modules/loader-runner/lib/LoaderRunner.js:111:13)

From the stacktrace I can see the warning is caused by postcss-loader