The mind will always try to justify everything, even if you're standing in front of a blue elephant, so you need to fully analize your surroundings, make your reality checks, but what I think really helps is to remember how you got there, what where your doing before?, like a very quick flashback of your day.
Last week I was in a dream, I made my reality check, everything seem normal to me... I was about to continue with the dream when I remembered that the mind will always justify eveything, so I went back again, analized surrounding, tried to remember how I got there which becomes difficult, and so I noticed the things that weren't right, like being with people I haven't seen for years.