There are three clastic sedimentary rocks that are composed of very fine particles (clay and silt). The general name for these rocks is mudstone, however we can classify these rocks even farther. Feel your rock with your fingertips and see if it feels smooth or gritty.  Some geologists will even put a tiny fragment between their teeth and grind on it a little to see if it is gritty or smooth. If you determine your rock feels gritty, take a closer look at it with your hand lens.  You may be able to see tiny grains sparkling in the light. If you determine that your rock is smooth (or composed of clay size particles) then look to see if it tends to break in layers or if it tends to be more blocky in appearance.