The only sign that you need to throw it out and start over is if you see pink or orange streaks in it.
Starter is actually pretty hard to kill as long as you don't subject it to extreme temperatures. The only sign that you need to throw it out and start over is if you see pink or orange streaks in it. That means that your starter has attracted unfriendly bacteria or mold and isn't safe for you to eat, so throw it out. Same thing if it's visibly fuzzy green. Yeah, that's also bad. Dump it.