You'll want to start by finding lots of pictures. Watching on tv can be helpful but it's nice to have a still image to look at. If you can't find a good picture online, you can take your own screen caps or check out this website.
Things to look for:
- full length shots
- front shots
- back shots
- side views
- detail shots
- shots where the garment is moving
- officially released shots
Movement shots can help you determine a bit more about the fabric used in the garment. This one has lovely movement. This shot also has a nice crisp shot of the belt and buckle. The belt is exactly the right length (no room for post dinner expansion!) and has a square buckle with and oval hole. I don't see a prong. It doesn't look to me like a self-fabric belt but it could be a nice green leather.
This picture is not a good one to use because it's very dark and we can't see much of the skirt.
Backs of garments are the most difficult shots to find. This one is has loads of outfit details available. Mary's skirt has 6 buttons on the back. I suspect they are decorative as I don't see any button holes, but they may be there.
Another difficult thing to discern is closures. Cora's skirt in this shot has an obvious closure in the back that looks like a zipper. It may also be a more period snap placket. If the buttons on the back of Mary's skirt are decorative, her skirt must close on the left. I haven't found a nice shot of the left but front and right side shots show no closures. Edwardian closures are often tricky with each layer having the closures somewhere else to prevent one spot from being too bulky. The other option is to just say forget it and put your closure in where ever you want.
Candid shots are great for giving a different view of things. This skirt looks much less full in this shot. (Plus we get to see how they have perfect hair all the time!)
Official release photos are nice because they are crisp and often much larger than screen shots so you can zoom in and see all the details. Look at all of the types of lace on this blouse! I must learn some heirloom sewing techniques!
You can apply these techniques to any period (or vintage) movie/show that you like.