you can go outside and do a burn test to determine whether the fibers are animal, vegetable (like cotton) or synthetic.
http://www.fiber-images.com/Free_Things/Reference_Charts/free_reference_charts_fiber_content_guide.html
You can put it into the bathtub with cool water and wool safe detergent like Eucalan (available at craft stores that sell woolen yarn) or supermarket Woolite. They will both felt wool if you agitate the water, so just lay the Afghan into it and let it soak for a couple of hours. Rinse twice in plain room temperature water, roll in towels to squeeze the bulk of the water out, and dry flat away from direct sunlight. On newspaper in a carport is ideal, the car won't mind being outside and you don't lose floor space.
Chances are good that it's acrylic. Those you can throw into the washer and dry in the dryer, it's what I knit my Afghans with just for that reason. I do find that using extra liquid softener keeps it from feeling like plastic. Since everything else in here reeks of sheep and wool, I've fooled a lot of people with them if they're super soft.
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