SMOAK Fireplaces are now providing free smoke testing for your home. (Subject to availability and catchment area of course!)
A Smoke test can determine if a chimney is well-sealed. Hot air is used to start the convection of air up through the chimney, then a small smoke block is lit in the fireplace and the smoke is drawn up.
If a chimney is in poor condition, then smoke may leak through the brickwork - inside or outside. the other day I did a smoke test at a customers house and smoke was pouring out of holes in the brickwork at the top! You can watch a time lapse video of the chimney re-build too!