Obviously the easiest way to know that a chemical is present is to see it.  You should be alert to spills and leaks of chemicals in your workplace.  You may also be able to feel a chemical that you come into contact with .  Some chemical vapors can irritate the eyes and throat.  Solvents can make your skin feel cold, which is a giveaway that your chemical gloves may not be protecting you like they should.  Acids and bases,  which are corrosive, can cause a burning sensation on the skin, in the eyes, or in the nose or throat if they are inhaled.  Most chemicals have a noticeable odor which is often described in the MSDS.  Be careful though.  There are some chemicals that have odor thresholds higher than the concentrations that can produce health effects.  You can’t rely on your nose to protect you from these chemicals.  Check the MSDS Health Hazard section for information on signs and symptoms of chemical overexposure.  If you are working with chemicals and experience these signs and symptoms, you may be overexposed and should stop what your doing and figure out a way to reduce your exposure.  Again your EHSC, supervisor, HSO, and RHSM can help.  In some cases we may monitor the airborne concentrations of chemicals in your work area.