Stage 1- It is your standard love with no sort of issues hence referred to as safe
Stage 2- These ones often will stalk their loved ones and will rarely take action against anyone else but they are known to be deceptive and take actions to manipulate others to their advantage
Stage 3-They will directly take action sneakily and eliminate any "obstacles" to their loved ones on a bigger scale than stage 2
Stage 4-They are unstable and are known to actively hunt for anyone who is unfortunate enough to meet her crosshairs.
Stage 5- There are very few who can reach this status and they are very dangerous and lethal to everyone around them including their love interest.
Stage 0-They are your waifus so that means you have to deal with the consequences of your choice

Tips:
Stage 1-There is almost nothing to worry about but they might become a stage 2 if they have any mental conditions or if they become very clingy to their target to the point of where they are spending all of their time with their target
Stage 2-Look for anything that goes missing for any reason (socks, toothbrush, underwear. Etc) and stay cautious with any notable changes in personalities.
Stage 3- Make sure you are always on guard and look for any disappearances of people you interacted with. If you do notice then make sure the authories are notified and to put some security onto your home and get firearm practice (if possible)
Stage 4- get as much distance from the area as possible and get a weapon you can effectively defend yourself with and don't hesitate to kill the one who is after you. (If they catch you....

You're a dead man)

Stage 5- use any advantage to beat the yandere (stealth, firearm proficiency, Physical strength, anything) if all else fails then make sure that your body won't be ever discovered/retrieved.( Suicide bomber is not ideal as some yanderes will keep your corpse to themselves)

Good luck out there