How Do You Know If You Have Creosote Buildup?

How Do You Know If You Have Creosote Buildup? Reduced drafting in fireplace. Less ash and more wood in fireplace after a fire. Black soot in and around fireplace. Very dark smoke from chimney. You can visibly see more than 1/8 of an inch of soot build-up. How can I reduce creosote buildup? Only burn