I think he just trusts you, that you don't cheat and don't abuse the rules.
But if you want to make sure, that you spend 48hours, you can track the time you spent by yourself by enabling a stoper when you work and stop it when you stop. Then save these times somewhere and just sum them up. If it's below 48, you have time. If it's in the 48 hours frame or you have a little bit more than that, that's it.
Also, you can just record it(well.. 48 hours of video footage might take some disk space) or stream, as you mentioned.