Place some masking tape on the face of your driver when you're at the driving range. The marks left on the masking tape after striking the ball will show you whether you're hitting your ball squarely on the face of the club, indicating if the club is open or square at impact. Once you understand how and why you slice your ball, with practice, you will consistently hit straighter shots. Knowing how to slice when needed is an advantage. Knowledge is power. You'll discover that there are benefits to an intended slice, especially if you want to gently fade a ball around an object like a tree.
Curing a golf slice takes time and practice. It's also advisable to check your swing technique at home in a mirror. Practice your back swing and look in the mirror to see if in fact the shaft of your club is parallel in your back swing. Then, as you descend the club, check to see if the end of the club is pointing at or near where the ball would be on your down swing, checking also if the club face is open or square.