You can't step through the code, but you can display every part of your formula one at a time.
For example
sum({SMKPLUDetail.Current Stock}, {SMKPLU.Product No}) < 1 then 0
else if sum({SMKPLUDetail.Current Stock}, {SMKPLU.Product No}) > 0 then sum({SMKPLUDetail.Current Stock}, {SMKPLU.Product No})
else if {SMKPLU.Product No} = 1348 then 0;
sum({SMKPLUDetail.Current Stock}, {SMKPLU.Product No})
will show you the value in that sum, but if you put
{SMKPLU.Product No} instead on the last line it will tell you which product you are calculating on.
This is slow and annoying but does allow you to debug.