Some roof spaces are more easily converted than others and a full assessment requires professional help. But the untrained eye can often work out the basics. Go up into your loft space and take a look around.
Is there enough headroom at the centre under the ridge? You ideally need 8ft (2.3m) clearance, although you may find that you can still get a useful room from as little as 7ft (2.1m). A quick check with a tape measure will suffice.
Also try to assess whether your roof was built using trussed rafters
or purlins, as the latter is much easier to convert into usable loft
space.