This can be tricky for homeowners to determine. Its best to have your chimney cleaned and inspected by a chimney sweep if you suspect one has occurred.
Depending on the amount it is being used, we encourage people to have it cleaned at least once a year. If it is being heavily used then every 6 months.
Spray water on your chimney. If the water beads off then it has been waterproofed, if not then a coat of waterproofing should be applied. Also do a visual check periodically, looking for cracks or creosote seeping out through the bricks.
IF your chimney has cracks forming, creosote staining and/or water is not beading off the bricks. If you have a strong smell of creosote inside of your house or there is any signs of water penetration then it definitely should be looked at by a professional. Don’t put off repairing your chimney, like anything the sooner you deal with it the lest costly it will be.