Essentially it's very tricky to tell, even with a CT scan (I think that the definitive diagnosis is made with an MRI scan - and with the NHS in the state it is good luck with getting one of those...) If it is a clot then speedy treatment can limit or even reverse the effects, but the same treatment can easily cause death with a bleed - so you need to be sure of your diagnosis, and there are few places in the UK who are happy doing that.