The first sign of SSD failure is if you get a warning from the "Manage Disks and Volumes" feature in Windows. This tool shows you the overall health of your SSD, its current temperature and even its lifespan.

You can open this tool by typing "disks and volumes" in the search box from the taskbar. This will take you to it in the system settings. Alternatively, you can use the firmware that comes with your SSD (e.g. Samsung Magician, Kingston SSD Manager, etc.) to check SSD health.

It is really useful that you first check for signs of SSD failure either using your manufacturer's firmware or Windows' "Manage Disks and Volumes" feature. This is because motherboard, RAM, driver issues and loose even loose SSD cables can mimmic the signs of a bad SSD. 

However, by using your manufacturer's software or the Manage Disk and Volumes feature, you can jump to the heart of the problem immediately.  Many storage devices use Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T) to accurately check SSD health. Those two features take full advantage of that technology,

Other signs to look out for are:

1. Crashes when your computer is booting (May be due to bad sectors affecting the master boot record)
2. Slow performance
3. Problems reading and transferring files
4. Problems detecing the SSD (may also be due to poor cable connections or incorrectly assigned drive letters)