If you are facing the task of separating an object that has been previously glued together, it is important to know that adhesives have certain characteristics that make them adhere and also fail. Technically speaking, adhesives cause bond between substrates, therefore what you are trying to do is make the adhesive fail to break that bond.
First thing you need to do is find out what kind of adhesive you are dealing with. If it is a silicone based adhesive
– commonly used for caulking- you are in luck because it is one of the easiest adhesives to get to fail. Just apply some heat to it and then you can peel it off. Try using a hair dryer.
If you are dealing with a stronger adhesive like cyanoacrylate, better known as crazy glue, try using some acetone which is the main ingredient in nail polish remover- make sure you are in a well ventilated area otherwise the fumes will get to you, so open the windows and let some air in. If you just spilled the glue, pour some water on it. It will cause the glue to cure and solidify. Then you can use some tool and scrape it off. Do not use paper towels to wipe it this could cause fumes, believe me this is something you do not want to happen, cyanoacrylate fumes are irritating. Porous surfaces are more difficult to remove glue from so it might take you longer to get the glue to fail.
To remove epoxy glue, use some acetone and follow the precautions previously mentioned. Let the acetone sit for a few minutes, this will cause the epoxy glue to soften making it easier to remove. Proceed to scrape it off with a tool.
Learning what type of glue you are dealing with and with some patience you should be able to remove that stubborn glue with out a problem. Always keep in mind the surface that you are working on and how delicate it is to avoid permanent damage while trying to remove the glue.