Google provides documentation for Dialogflow webhooks here, which include this sample webhook to inspect parameters and dynamically create slot filling prompts:
exports.dialogflowFirebaseFulfillment = functions.https.onRequest((request, response) => {
const agent = new WebhookClient({ request, response });
function flight(agent) {
const city = agent.parameters['geo-city'];
const time = agent.parameters['time'];
const gotCity = city.length > 0;
const gotTime = time.length > 0;
if(gotCity && gotTime) {
agent.add(`Nice, you want to fly to ${city} at ${time}.`);
} else if (gotCity && !gotTime) {
agent.add('Let me know which time you want to fly');
} else if (gotTime && !gotCity) {
agent.add('Let me know which city you want to fly to');
} else {
agent.add('Let me know which city and time you want to fly');
}
}
let intentMap = new Map();
intentMap.set('flight', flight);
agent.handleRequest(intentMap);
});
My guess would be to add
console.log(agent);
right before defining the flight function, then checking the logs to see which objects agent contains, then adding iterations of console.log(agent.fakeObjectName) until you find the information you're looking for.
If you're following the deployment process recommended in Actions on Google's Codelabs level 2, your logs will show up in the Firebase console, like this:

Hope that helps!