diff --git a/Debugging.md b/Debugging.md
index f4439f3..81d7583 100644
--- a/Debugging.md
+++ b/Debugging.md
@@ -8,6 +8,13 @@ Enable [logging](Customization#--log-filefile----log-file-appendfile) and
 
 `-log-file=/tmp/ccls.log -v=1`
 
+To figure out the compiler command line used to parse your source files:
+
+* Find the line below `indexer* ... parse`. This is used be the indexer.
+* Find the line below `preamble ... create session for`. This is used for completion/diagnostics.
+
+If there is some diagnostics problem, change the first argument to `clang`, append `-fsyntax-only` and run the command in a shell to reproduce.
+
 ### stderr
 
 You can find stderr output in: