It's very normal for your baby to lose weight in the first few days of life. But after that once breastfeeding's established, either breastfeeding or formula feeding, your baby will start to gain weight normally.
The best way to tell your baby is getting enough to eat is actually to look in their nappies, babies should be having regular wet nappies and pooey nappies. or with breastfed babies they may not have their bowels open for two to three days.
But when that bowel is opened the stool should look loose, seedy and a yellowy mustard colour. When a baby is satisfied and has finished a feed the baby will just come off the breast or will push the bottle away and that is them saying I've had enough and I'm happy with that feed.
If your baby is happy and satisfied, and relaxed after feeds, and having nice naps and wakeful times, and your baby is gaining weight, then your baby is doing really well and you don't need to worry.
About video: Health visitor Penny Lazell talks about how to find out if your baby is getting enough milk. http://www.nhs.uk/video/pages/is-my-baby-getting-enough-milk.aspx