The strips or a blood test are the best methods. If you are water fasting, you should be in ketosis in about 48 hours. If you are hypoglycemic or pre-diabetic you may notice that ketosis will occur earlier, but this is not a good sign and indicates a health problem. I suffer from insulin resistance, Metabolic Syndrome, and usually go into ketosis in 24-36 hours when water fasting.
With Master Cleanse, juice fasting, and extreme low carb diets the onset of ketosis will vary depending on how many carbs you consume, and on your glycogen reserves. Also, if you continue to indulge in heavy exercise, the muscles will begin to break down to release glucose to keep functioning. Muscles need glucose, they cannot use ketones as their fuel. People who exercise need to include some form of Sugar in their fast to prevent muscle wasting.
Despite all the Atkins hype, ketosis is not a magic bullet. Yes, it does indicate that fat tissue is being broken down, but it also produces a strain on the kidneys and makes the body acidic. Anytime you are consuming fewer calories than your body requires, fat will be broken down and used for fuel. But when fat is rapidly broken down a lot of stored toxins are released into the blood stream. The real key to a successful fast, especially when rapid weight loss is the key goal is balance. The healthiest way is too use a nutritious juice fast most of the time and to Water Fast one or two days a week. This method will keep nutrients and minerals in the body with very few calories, the digestive system can rest, and muscle wasting will be reduced or eliminated. Plan your Water Fasting days when you can rest and pamper yourself. You can also do a cleansing diet like the six week "Eat To Live" plan from Joel Fuhrman and throw in a fasting day once or twice a week. You will probably go into mild ketosis, but will not put as much stress on your body.