You can tell if your baby is getting enough milk by looking at how much they are weeing and pooing, and whether they are gaining weight. If they are producing the expected amount of wee and poo, and their weight is increasing and approximately following their centile line on their growth chart, then they are getting enough milk. You cannot tell how much milk you are producing from how full/empty your breasts feel, or how long your baby feeds for.
Sometimes breastfeeding mothers worry that they are not producing enough milk for their baby. However, most mothers ARE able to produce enough milk, and if your milk supply does drop, you can usually increase it again with some simple measures, including adjusting your positioning and attachment and feeding your baby frequently and responsively.
If you are concerned that your baby is not getting enough milk, speak to your breastfeeding supporter, midwife, health visitor or GP.