How can you tell if a gap is wide enough to safely change lanes?

When changing lanes to the left or right, make sure you have a minimum of a car and a half or two full car lengths behind or in front of the spot you wish to change to depending on your speed.

When changing lanes to the left or right, make sure you have a minimum of a car and a half or two full car lengths behind or in front of the spot you wish to change to depending on your speed. Use your reference points like your hood and windows to judge distances and make sure you practice on your own time switching safely between lanes. Make sure to use your side mirrors when changing lanes to check your blind spots adequately.

Use your rear view mirrors to check the distance between you and the car behind you. Remember to check the blind spot by glancing over your shoulder to judge the space between. Then make sure to indicate whether you taking a right or left lane to pass the car in front.

Eyeball it by using your reference points. If it even slightly looks like your car is too close, then you are and you shouldn't take the chance on it. Rear view mirrors and reference points like center of hood and middle of windows are the best option.