Toyota Lucida starter seems to be spinning too fast

2 2 diesel lucida was running great but on tick over on my drive it just cut out and now won't start .at first thought it was lack of fuel but it's pumping through ok .as it's turning over there is some engine rock but not much ' checked fuses all good its a new powerful battery and iv had the starter checked and it's fine "" but my alternator is not giving out more than 11.5 volt's but the car should still start with a full battery I know" could be a snapped belt I know or timing .how can I tell 100% that its a snapped belt ' thanks