Unfortunately, Retro68 doesn't include a proper debugger.
Common Problems
If you find you don't even get to your main function, check your program's memory size.
In the Finder, choose Get Info for your program and increase the "preferred" and "minimum" sizes. If that helps, include a 'SIZE' resource in your program to set the "suggested" and "minimum" sizes automatically.
If that's not the reason, it's very likely that you've run into a bug in Retro68... (open an issue!)
MacsBug
The most important tool for debugging classic Mac applications is MacsBug:
http://macintoshgarden.org/apps/macsbug-663
It will run on real Macs and in minivmac; I haven't had much luck with Basilisk or SheepShaver.
MacsBug is a low-level debugger. This means disassembly, hex, black magic, and a command line.
There should still be tutorials floating around on the net, but to get started, you absolutely need to know two commands:
g - for "Go". Continue execution.es - for "Exit to Shell". (Try to) exit the current program and get back to the finder.
MacsBug will be triggered automatically whenever there is a crash in your program.
In case of simple bugs, you might even see a function name displayed. (try the `sc' command to get a stack trace; it sometimes works!)
You can also deliberately break into the debugger using two functions you can call from your program:
Debugger(); - enter the debugger.DebugStr("\pHello, world."); - enter the debugger and display the string.
Have fun...
fprintf or std::ofstream
Retro68's standard library support has huge holes, but you should be able to write log files to disk and see how far your program got.