When you are trying to perform a trace from a browser environment in which you don't have access to tracer, (such as a portal or mashup in which developer tools are not visible), you will need to login to a different browser session with developer access and use the remote tracer. However, how do you find out the thread id to pick in the remote tracer so you can trace it.
See Screenshot for reference
In a different browser (Directions and screenshots assume Chrome), login in with a developer role and open tracer. Click on the remote tracer option and see the list of all the threads available in the drop down. Which is the thread you want to trace?
Go back to the other browser whose pega session you are trying to trace.
Using Fn. F12 open the browser's debugger.
Go to the network tab. and select any of the lines that are displayed.
Click on the Cookies tab and and view the Pega-RULES cookie. It will contain the thread information.
Match this to what you select in the remote tracer drop down.