My friend has been very sad lately and I think she might have depression , bbut dont know what the symptoms are.

There are a lot of different symptoms. they can be polar opposites for example;
some people quit eating, or eat very little other people eat a lot.
some people get really quiet, quit going out, and avoid human contact; other people party it up!

I would just talk to her. Ask her how she is, tell her you've noticed a difference in her, and you're concerned, and if she needs a friend you'll be there!

Another thing to do is it to google mild depression. Read a little bit about it! So you'll have a better understanding!

Good Luck!

wanting to be alone, and to not hangout
eating very little or ALOT
looking sad (sometimes)
tired or sleepy
easly irritated, cranky

these are just some, but some people can be depressed and not even show any of these symptoms. it really depends on the person

not wanting to do anything, sleeping alot, feeling alone, shyness, um not eating and more but those are some of them

lack of sleep, lack of energy, general bad mood.