I think your question is confused as avast doesn’t run in safe mode.
So I’m guessing that you mean you asked avast to do a boot-time scan ?
If so you should see it run before windows actually starts (see example images, will differ on your OS and these are old, but you get the idea), so you should know it is running, did you see that ?
If it did run and complete check the C:\ProgramData\AVAST Software\Avast\report\aswBoot.txt using notepad.
To add on what David R stated, if there is no infection after the boot scan it will continue to start Windows. Initially as the pics show by David R you can observe the start of boot scan and the percentage completed. Prior to scheduling a boot scan you can set the parameters for the scan and what you want Avast to do as you can see in the pics.