Apple /// not booting

Hello everyone,

I recently got hold of an Apple ///, however it doesn't want to boot any Apple /// software.  It will however boot nearly all my Apple ][ software using Apple ][ emulation. 

Basically it freezes on the Apple /// SOS welcome screen, see image below. The disk drive also stops at this point. Someone mentioned to me that it may be a RAM fault as the Apple /// doesn't have enough memory to completely load SOS. Is there a way of checking how much memory there is?

Thanks in advance

Max