The vast majority of the 100,000 or so known species of fungus are microscopic moulds. Like the fleshy fungi, mould identification is based on the fruit bodies and spores. The difference is that the fruit bodies are much smaller, so a microscope is essential, or at the very least a good quality hand lens. You'll soon realize that moulds rule the world!
One way to collect a sample of mould is to press a piece of clear sticky tape against it. Place the tape sticky side up on a microscope slide and examine it under a light microscope. Hyphae, fruit bodies, and spores will all be revealed.