Unlike the plagioclase group, which varies in composition, potassium feldspar has the same formula, KAlSi3O8. (more below)
Potassium feldspar or "K-feldspar" varies in crystal structure depending on its crystallization temperature. Microcline is the stable form of potassium feldspar below about 400° C. Orthoclase and sanidine are stable above 500° C and 900° C, respectively, but they endure as long as they need to at the surface as metastable species. This specimen, a phenocryst from a Sierra Nevada granite, is probably orthoclase.
In the field, it usually isn't worth figuring out the exact feldspar you have in your hand. A true square cleavage is the mark of K-feldspar, along with a generally less translucent appearance and the absence of striations along cleavage faces. It also commonly takes pinkish colors. Green feldspar is always K-feldspar, a variety called amazonite. Field workers generally just write down "K-spar" and leave it at that until they can get to the laboratory.
Igneous rocks in which the feldspar is all or mostly alkali feldspar are called syenite (if quartz is rare or absent), quartz syenite or syenogranite (if quartz is abundant).