Re: how do i know if a system restore point will work?
There is no way you can know ahead of time which restore points will work because that would require knowledge of the future. Any restore point should work but there are situations where they do not. System Restore works best if it is used as soon as possible after a problem occurs. The further back you go the more likely that a system restore will fail or have undesired effects.
When a system restore is attempted (except in safe mode) it will itself create a restore point to allow an undo of the restore. This will likely delete one or more older restore points. Once a restore point is deleted it cannot be recovered. System Restore is most reliable in safe mode but be aware that if it does not work as expected it cannot be reversed. This is because restore points cannot be created in safe mode.
Do not expect a system restore to recover user data as it was specifically designed not to do this. That is what backups are for.