Yes, there's a ping function inside Colyseus:

v0.15:

import { Server } from "colyseus";
import { WebSocketTransport } from "@colyseus/ws-transport";
const gameServer = new Server({
    transport: new WebSocketTransport({
        server: http.createServer(app),
        pingInterval: 5000,
        pingMaxRetries: 3,
    })
});

before v0.15:

import { Server } from "colyseus";
const gameServer = new Server({
    server: http.createServer(app),
    pingInterval: 5000,
    pingMaxRetries: 3,
})

Then you can send something by client, and timing yourself.