In my own field, the person who had done most of the work (including probably having the idea, doing some analysis, and writing the paper) would safely be the first author. The person presenting the work could be any of the authors (but preferably it would be the first author), and normally their name would be __ underlined __ on the title slide. In some unusual cases, a non-author might present (eg visa problems, illness, etc). In such cases, a special note would be needed on the title slide. In cases where the first authorship is confusing, eg because one person had the idea and did all the analysis, but the other person did all the writing, it is less clear cut, and you need to discuss.