You can also flip the switches to check your chance if enemy uses IRST or a Radar which will usually be a lot lower. In the bottom right part of the UI you can see the chance of executing a Sudden Strike with currently selected ships - the number maxes out at 99%. Ship speed of 300km/h (at night, or during daytime in bad weather) and 400km/h (during the day in clear weather) is enough to guarantee sudden strike.
There is also a small degree of RNG involved in how fast you are spotted. City garrisons rely on visual identification only with a range of 100km during the day in clear weather and 75km during the night or in bad weather. To successfully execute a Sudden Strike the Alarm must not be currently active and you need to reach the city and start combat before the enemy reacts to your presence. Sudden Strike (also called Silent Strike) is a mechanic which allows you to take over Cities without alerting the enemy to your presence.
