Not many flavorings used to make eliquid still contain diacetyl. For those that do...most manufacturers and online websites selling those flavorings...will say so in the description or ingredients list of the flavoring. There are quite a few flavorings though, that do contain just trace amounts of the diketones acetoin and/or AP (acetyl propionyl). That's usually also either stated in the ingredients list or the description of the flavoring, or is indicated by a specific symbol (usually an *). That's helpful info for those of us who DIY our own eliquids. Many times there's also a "diketone free" version of the same flavoring, that doesn't have them in it. So we can choose which to use for our own eliquids.
However, whether or not the shop where you get your eliquid will state whether or not the ones they sell contain any of the above...you'll have to ask to find out. Many times though they may not even know if they do, unless the eliquid was made by them.