Do a picture/image search for guano or more specifically 'bat guano' I did one quickly and didn't find much in the way of fresh/unprocessed images. However guano has a distinctive appearance and a distinctive smell. If you see it you will know what it is. Cat piss is very potent - one male cat (unfixed) could have produced enough urine to make a space smell like 20 tome cats have lived there for years. Raccoon piss is also as potent. Squirrels also have pungent smells. Rats mice also smell... While a trained nose may tell the difference of the fragrances, most people smell urine and only urine. If it smells like urine it most likely is urine. No matter the source. Look for spore (feces) and you can tell which critter left it behind. Your best bet may be to call and exterminator and let him/her find the spore and tell you what critter is the cause. That way you know if this is an internal problem (rats and mice go up the inside of walls) or an exterior problem (raccoons and squirrels and bats come in through openings like vents) The problem is two fold - you need to locate the spot(s) and do some deep cleaning - if this is not structural (beams, joists, etc) that is affected you might want to just rip it out and replace with new. If it is structural you need a 2 cup of bleach to one gallon of water solution to spray on to the affected area(s), let it stand for 15 minutes, rinse and repeat and repeat and repeat. If it is untreated wood the wood will lighten. If it is treated/finished wood then you need to strip the finish to get at the soaked in wood. Since this is an attic I will assume you don't have to worry about things like hard wood floors. Once you have treated the area a few (can be a dozen times depending on how long and mow much urine has been left behind) you will need to let it all dry out - I suggest opening up the attic vents and using a heavy duty fan to move air through the area. Once it is dry then you paint it with three coats of paint. If the insulation has been compromised, you need to replace it. You will need to clean the area beneath - if this is drywall/plaster you can not soak it, but it should be checked for mold/mildew and you need to look up from below to see if the urine has left a mark on the underside (ceiling) of the room(s) below. if it has then you need to replace that plaster/drywall soonish. If you have exposed floor insulation you might need plastic bags and a shovel. More often than not the blown in type is on the floor of an attic. Rats, cats, bats, squirrels, raccoons - and many other animals find their way into attics. you need to locate the holes that allow exterior critters in, either cover with screen (metal) on vents, or repair the holes. Metal flashing is perhaps best, especially with mice/rats who will go back to the same location to gnaw through again. Thus a replacement with wood followed by aluminum flashing on the side to prevent the rat/mice from coming back. This could be a fairly expensive process, especially if you will need to replace insulation. IF so then you might want to consider installing better/thicker and more insulation and see what kind of tax rebate cutback you can get from the power company and the state/local government you have. While they will not cover removal of soiled insulation, home energy improvement projects are covered.