I think the trick is to know how much of that is VATable income (not just income). There's no way to record if a transaction is VATable if you're not VAT registered unfortunately, but hopefully giving yourself an idea of the overall income would be a good way to check.
What I'd recommend is having a look at the custom report option. This would allow you to use certain nominal codes (say 4000:4999) to calculate the income in a period. Setting to a bar chart on a month view for example would also generate a table at the bottom with monthly totals, and an overall total:
I hope that helps!