What do legit journal/conference/review invitations look like?
The #1 criterion is: they come from someone whose name you recognize as a respected colleague in the field. Or, at least, a conference or a journal that you know already.

The #2 criterion is: they look like they are not written automatically. Compare


  Dear Arthur, I have seen your very interesting article on shiny rocks and I would like to invite you...


and


  Dear A.Uthor, I have seen your very interesting article Shiny rocks in South African Mines: a Case Study and I would like to invite you...


The second one seems auto-generated, since they copied over your name and title verbatim. The first one looks like someone went through the effort of paraphrasing the title into a real English sentence.

In my opinion also putting a lot of emphasis on proceedings, Scopus and ISI-indexed journals and impact factors is a red flag. This is something I would never mention if I were inviting a real person to a conference.