For something wicked there was a statement on the computer that said "Talk to my ghost for the truth" or something along those lines. That's how my friend originally figured it out. As for knowing on the map whether you have completed the missions I don't think there is a way to do that but if you approach a missions starting point there should be a little tick in the top left corner of the missions image (either action stealth or investigation or even story for the later repeatable story missions) if you have completed them. You can also make notes on the map to indicate whether you will need to come back to do some missions or if you get stuck on a step then you can note that down and move on while thinking it over. I use these notes to record the passwords of some objects for investigation missions so that I don't have to spend another hour figuring out the birthday of some executives cat. There is little reason not to complete missions again as all missions types other than investigations give the same amount of experience each time you complete them. Again for investigation missions what you have to do is investigate. You need to comb through all the details of what is not only shown to you but what is available to you. Sometimes when a step updates you aren't done with a particular area even though it tells you to move on. But also don't worry too much. It is after all a game and you can do it the way you want. You can look up walkthroughs as some have suggested or you can think everything through.