In some instances, a recycling logo and code may not be marked on the item in question. Under these circumstances, you will have to make an educated guess or do a bit of research. There are a few things to keep in mind that may help you determine the exact plastic you're dealing with. If the plastic is hard like a bucket or milk crate, it is likely to be polyethylene or polypropylene. These plastics are nearly impossible to glue unless you choose an adhesive that is marked specifically for use with polyethylene or polypropylene.  Clear, brittle plastics like imitation glass or CD cases are usually polystyrene. A poly cement will work best on these. Logos are made of ABS. If the mystery plastic feels similar to a Lego block, you'll want to use an epoxy. It is important to note that solvent-based adhesives can warp and change the surface of the plastic. This is something to consider if appearance is important.