Is there a wise way to debug a large code?

[CuriousOne]

Ok, here is the part of code which reads the keys:

adcin 7,ard
if ard<130 then 'if any button is pressed
ticker=ticker+1 'increase value and wait - debouncing
pause 1
endif
if ticker>150 and ard<100 then  'if left button is pressed then next menu item
menuitem=menuitem+1
ticker=0 'reset debouncer
endif
if ticker>150 and ard<130 AND ARD>100 then  'if right button pressed, change the value
CVLADI=CVLADI+1
if cvladi>ulim or cvladi<dlim then cvladi=dlim
ticker=0

here is PCB