help - how to tell apart signed build from unsigned build ?

[Mikus Grinbergs]

 From reading the wiki, I am assuming that 'olpc-update --usb' can 
install a new build on a secured XO (G1G1, without developers key).

Suppose I am handed two USB sticks.  One contains files os1.toc and 
os1.usb;  the other contains files os4.toc and os4.usb.  If I'm told 
only that one of these USB sticks contains a signed build, and the 
other contains an unsigned build, HOW can I tell which USB stick I 
should use to use to install a *runnable* build on that secured XO ?

Thanks,  mikus