Generally, with RootKits and BootKits, it takes an experienced person to notice the signs or even to suspect that the system may be infected with one. You develop a feel for what you are doing and you learn what to look for in the logs of the programs you use to find and remove any Malware & such.
Because these items usually will do what ever it takes to hide themselves from being noticed and BootKits load before the Operating system itself. But there are tools out there which can be used to detect signs of RootKits.
BIOS infections can cause problems accessing the BIOS itself, it can cause video problems before Windows loads up and there are other signs too which may or may not be present depending on the variant.
Plus one of the biggest things is you get your system completely cleaned of any infections including any RootKits (not knowing of the BIOS infection), then next thing you know, shortly after your system is connected to your Broadband modem, it's infected again without your even doing anything except leaving it turned on for a few hours.
What happens is the BootKit will notice the MBR RootKit and possible Memory resident too is missing, so it'll connect online to download a new downloader module which in turn will download RootKit and any other Malware Modules which were removed.