A course usually has its golf cart rules on its scorecard, or posted in the clubhouse or near the first tee.
If you learn that the "90-degree rule" is in effect, this means that you can take the cart onto the fairway, but only at 90-degree angles. Stay on the cart path until you are even with your ball in the fairway, then drive straight across to the ball. Hit the shot, then drive straight back to the cart path and proceed forward.
If you are told that the cart path only rule is in effect, then keep your cart on the designated cart paths at all times.
Most courses, on most days, do allow golfers to driver their carts onto the fairways. Ask in the pro shop at check-in what the cart rules are. And, as stated at the beginning, if you are unsure then just keep your cart on the path.
If you are walking and using a pull cart, do not take the pull cart onto the putting surface or into hazards.