Yeah usually by nappy output but also see if there’s a weighing clinic near you. I always worry about this but I got my first weighed at the clinic monthly and she was always gaining weight. Have been worried about my newborn but weighed him at his 6 week check and he’s also gained so milk is enough. Try asking you health visitor or looking up the children’s centres near you
Could also be worth reaching out to your local breastfeeding group or ask your HV to signpost you as they can help you with feeding
I think there’s a cluster feeding growth spurt around now so probably totally normal. If baby is having wet nappies you are doing great 😊
Output is the biggest indicator of input. https://laleche.org.uk/is-my-baby-getting-enough-milk/
Usually by output so if she isn't producing wet nappies ( as many as normal for her) or her weight drops x