Here are some signs that your doe may exhibit when in heat. Some does will have what is considered a silent heat and not show any outward signs that she is in heat. This makes hand breeding difficult so running her with the buck for a month ends up being basically your only option for getting her bred (he knows when she is in heat, even if we can't tell!)
1) Flagging - this is when your doe wags her tail usually vigorously and more often then normal.
2) clear to white discharge - noticeable on her vulva or on her tail, a transfer from contact.
3) Pink to red vulva - swelling in the vulva area which can be seen as pink or red depending on the degree of swelling.
4) calling - crying out more then usual, can even sound like a moan - mine usually scream
5) affectionate - being more friendly then normal, wanting you to stay with her and pet and scratch her - seems extra upset when you leave
6) mounting - yes a doe in heat sometimes will mount another doe. It can be flipped with the doe in heat being mounted by anther doe NOT in heat. So check for other signs on either doe to see which one is really the on in heat. The mounting can also be accompanied by blubbering noises and stomping.
Not every doe will show all these signs while in heat. Some may only show one maybe two while others may exhibit all of them. Again if you never sense a change in your does behavior she maybe going through what is considered a silent heat.
This is when a doe is willing to stand for the buck. This is when she is in her true heat and is ovulating. This can last for up to 24 hours. A doe maybe "in heat" for a week but only willing to stand for the buck for a short period of that time in which she can be bred. I find that it is common for a doe to be "going into heat" for 1-2 days be "in heat" for a day and then "going out of heat" for another couple days. This covers your week that you notice a difference in her vulva or behavior.
5 day heat
This is what a good book says about this common occurrence in does. Taken from Pgymy Goats Management and Veterinary care By Lorrie Boldrick.
A do will come into season [heat] and stand for the buck normally. We carefully enter the dates on the calendar and start planning for our babies in 5 months. But 5 days later, the doe is back in season and standing once again for the buck. He's happy, and so is she, but what does this do to our 5 month plans? Apparently the follicle produced with the first heat did not rupture (ovulate). But since it was ripe, it took only a few days for the hormones to build back up and prepare for ovulation once again. The majority of does will ovulate on this five-day heat and you should adjust your calendar accordingly. There is almost no chance that she would have ovulated at the first heat.
Dairy goats go into heat from September to February (with a few exceptions of does going into an early heat in August and a few with late heats into March). Pygmy and Nigerian Dwarf breeds can cycle once a month all through the year. I noticed though that their heats tend to be stronger in the fall months.