Blank page or error 500 on my host

My <stack> site shows a blank white page or an error 500 on <host>, and
works fine locally. The live URL: <url>

Help me find the real error:
- Where this host writes the error I can't see.
- How to turn on detailed errors briefly, for me only, without showing
  them to visitors.
- The usual causes for this stack on this host: a missing extension,
  a different runtime version, file permissions, a path that differs,
  a memory limit.

Once I paste the actual error, tell me the fix.