I have been fortunate enough to have a friend with a daughter a little bit older than my daughter and she has been generous in allowing me to go through the clothes her daughter has grown out of to let me borrow. The first few times I went through clothes I just grabbed a few (after all, we get so many 0-6 month clothing at baby showers) so I was able to remember which needed to be returned. However, once my daughter started wearing 12 month+ clothing it became much harder to remember what came from where because she wore that age range for so much longer than any other. My solution was simple: take pictures.
I took pictures of the latest batch so I can refer to them at a later time since my daughter will be wearing these clothes for a much longer period of time. Labeling clothing tags with a permanent marker is an acceptable alternative however some items don't have tags and it's possible the clothing might be accidentally stained or the tag may come off.
A few things to remember when you are given an opportunity to go through some one else's baby clothes:
-Always ask if they would like the items returned. They might have family that could use it even if they don't intend to have more children etc.
-Skip the ones they are most sentimental about. For example, a family made item that they really like... you might like it too but how horrible would you feel if you stained it or it got lost? It's just not worth it.
-Be sure to thank them for allowing you to go through the clothes.