Hi Murae,
You can find that within the legislation itself. For the EU stuff it'll be in the first section below the title/reference - if it's "amending regulation/directive..." then it's an update to that regulation/directive (i.e. all of the content of the original is still in place, except for the specific bits added/changed/removed by the amending article), whereas if it is "repealing regulation/directive..." then the things it is repealing cease to be in force and the new one takes over.
For example, everyone's favourite Food information to Consumers labelling regulation has the incredibly catchy full title of: Regulation (EU) No 1169/2011 of the European Parliament and of the Council of 25 October 2011 on the provision of food information to consumers, amending Regulations (EC) No 1924/2006 and (EC) No 1925/2006 of the European Parliament and of the Council, and repealing Commission Directive 87/250/EEC, Council Directive 90/496/EEC, Commission Directive 1999/10/EC, Directive 2000/13/EC of the European Parliament and of the Council, Commission Directives 2002/67/EC and 2008/5/EC and Commission Regulation (EC) No 608/2004
In this case it is amending Regulations (EC) 1924/2006 (nutrition and health claims) and (EC) 1925/2006 (addition of vitamins and minerals and similar to foods), so both of these regulations are still in force but are modified slightly by the new labelling reg.
It also repeals Directives 87/250/EEC (indication of alcoholic strength labelling), 90/496/EEC (nutrition labelling for foodstuffs), 1999/10/EC (nutrition labelling, amending an earlier directive), 2000/13/EC (labelling, presentation and advertising of foods), 2002/67/EC (labelling of foods containing quinine and caffeine) and 2008/5/EC (some extra labelling requirements not included in previous directives), and regulation (EC) 608/2004 (extra labelling requirements for phytosterols etc). In effect, the function of all of these has been combined and updated into the single new regulation.
There are of course also implications at a national level for EU member states, as the EU bits will be entered into local law via statutory instruments so you'd need to review those in parallel.
For the above example you'd want to look at The Food Information Regulations 2014 (SI 2014 No. 1855). Alas here in the UK it isn't laid out with a nice summary of repealed/amended acts at the top, so you'd need to look at either the table of contents (most UK regs on legislation.gov.uk have one) or the citation section (usually the first bit) to find out where you actually need to go to check. In the case of this example it turns out that Schedule 6 contains a table of revocations with details of whether this is the whole act or specific parts only.
That felt like it was a rather long-winded way of explaining things, but alas it isn't the simplest of areas.
Again using a trade association could help, as they should draw your attention to changes that are particularly relevant to your sector(s).