How to know which journal is open access?
You can tell whether a journal is open access from their website. "Supports open access" doesn't immediately say if the journal is completely open access, but it does mean that you will be able to publish open access in it. In other words, it's possible the journal practices the hybrid model where it is subscription, but offers open access to whoever wants it.

For Discrete Mathematics itself, you linked a ScienceDirect webpage. ScienceDirect is not the journal's publisher - it's just a database. What you really want is the journal's webpage itself. Clicking on the "Supports open access" tab on the left leads to this page, which tells you everything you want to know. This is a hybrid journal alright, and if you want open access the article processing charge is USD 1500 excluding taxes (potentially lower if you are funded by the Wellcome trust and Research Councils UK).

If you are unable to find whether the journal is open access on its webpage, assume it isn't. If the journal claims to be open access but does not list its article processing charges, do not publish there, since one of the key criterion to calling a journal/publisher predatory is whether its APCs are clearly listed.