A pair of pants can be made any kind of textile fibers. Majorly cotton and polyester are used in fabrics that is used for making pants. Depending on the design and end use, and comfort to wear apparel brands and garment manufacturers decide what type of fiber to use for a specific pants design.
If you just like to know what type of fiber is present in your trouser, you need to do the fiber content testing. There is some testing method that can be used to know actual fiber content. Even there is blend of fiber, that can be identified.
In two piece suits, wool fiber is also present in the trousers.