Here are some tips

-Press Ctrl+Break while the program is running to interrupt it. The current line will be highlighted. F5 will resume.

-The "immediate" window can be used to execute commands. For example, you can use it to change variables, call subroutines or print things.

If you want to watch an array, you can do something like this in the immediate window:

FOR x = 0 TO 50: PRINT Array(x); : NEXT

But you can be more creative.