The easiest method of identifying what kind of gilding one is looking at, is to look for the leaf lines; the spot where the leaves meet. In the above sample, the arrows are pointing at the lines of metal leaf (schlagmetal, dutch metal, composition leaf). When it is "gold" colored, it is a brass alloy, about 10,000th of an inch thick, and comes in a 5 1/2 inch square leaf. There are aluminum leaves; silver colored; copper leaf; copper colored, and various kinds of variegated leaf; multiple colors in one leaf.
Here, the leaf lines on matte, karat gold. The leaves come in slightly different sizes, depending on manufacturer, but are close to 3 1/4 inches square, and about 300,000th of an inch thick. This is the most common form of karat gold, though there is ribbon or roll gold, and there is a wide variety of karat weights and gold alloys, ranging from a white gold at 12K, to pure 24K leaves. For outdoor use; all those domes and statues, use a minimum of 23K. Another way of identifying genuine gold on picture frames, would be a distinctive difference between high spots and recessive areas of the frame, where the high spots have been burnished to a mirror like shine. This is called matte and burnish work, and requires the gold to be laid in the water gilt method, over a material called bole. Bole is mixed with glue, applied in multiple layers to the frame, polished, then gilt. When dry it is burnished with an agate stone, the most popular stone for burnishing. Bole comes in reds, yellows, blues, and blacks, with multiple shades in each color, and can often be seen where the leaf is worn, either through age or intention.
This is a brief explanation of what one is looking at; but this is a very ancient craft, and I touch on only the more common styles. There is gilding with tin foil; silver leaf lacquered to look like gold; bronze powders; bronze powder paint; and now, the non-tarnishing mica powders.
Size gilding, water gilding, glass gilding, engine turned gilding, and not least, gilding the lily. 8-)
And just to confuse the issue, there are many examples of lesser materials carefully finished to look like genuine gold.
Addendum: Metal leaf comes in other sizes as well, 6 1/4 square being the other common size.