You can check your balance on the website by logging in My giffgaff. You can also check it from your phone by keying in *100# (for airtime credit) or *100*7# (for goodybags). Other useful shortcodes are available.
Charges not included in goodybags
Understanding your allowance
goodybag and gigabag allowances are easy: calls are counted per minute (when the minute starts) and data usage is counted per MB.