It's not necessarily about looking for signs that they're engaged... it's more about looking at what we know and acknowledging that this relationship has the appearance of being very serious.
There are a number of posters on this thread who have been waiting for the engagement announcement practically since the rumours of a relationship first surfaced in October. There are also a number of posters who are determined to diminish this as no more serious than Harry's past relationships. But if you actually look at the facts, it's pretty clear that this is very likely more serious (and more mature) than his relationship with Chelsy or Cressida.
First there is the fact that Harry publicly acknowledge the relationship in November, before there were any pictures of them together, any evidence the rumours were true, or really anything more than rumours. This is unprecedented and it was a move he could have just as easily not done.
Then there's the fact that we've seen both Harry and Meghan make frequent trips to visit one another since the relationship became public - which, as much as air travel is easier than it has in generations past, you don't hop on a 7 hour flight just for a booty call. You can also add the fact that Harry broke protocol to briefly stop over in Toronto after his Caribbean tour. Sure he's impulsive, but again, that's not something you do for a casual relationship.
Then there's the fact that Meghan has drastically altered her public persona in the last few months. And given up not one, but two sources of income. We don't know the details about what happened behind the scenes with Tig, but we do know that Reitmans still wanted to continue their relationship with her, and she abruptly ended it. Not something you do for a casual relationship that isn't going anywhere.
I don't know if they're engaged now, or soon to be engaged or what. I'm not looking for signs for an engagement - I'm not expecting a publicly announced engagement until after Meghan's work in Suits is finished. But I definitely think that there are clear indicators that this is a relationship that both Meghan and Harry are taking seriously, and is in all likelihood headed towards marriage.