One of the more unusual things that happens when you step off the relationship escalator – whether polyamorous or monogamous – is the realization that you have to actually decide where the relationship ends up. 
We aren’t taught how to make that decision. It’s just assumed that relationships get “more” – more involved in each other’s lives, more entangled – until you’re married. It’s widely thought of as an automatic progression, hence the escalator.
That assumption has some really negative side effects. The worst is that every intimate relationship – heck, every first date! – is judged against the standard of “Will I join my life with this person’s indefinitely?” This is a crappy standard of a “successful” relationship. Some relationships are casual, some are short-lived, and they can each be meaningful (and successful) or not.
Anybody who doesn’t (almost exactly) match what you think you want doesn’t get to ride the escalator with you at all. And that’s cheapening your life. 
Once you’re off the escalator – and especially if you’re polyamorous – that preset default goes out the window. Do you decide to further intertwine your lives based on the length of the relationship? Or how you “feel”? Or some other metric? Should you even further intertwine your lives?
The signs I’ve been able to work out for myself are more like limits, or “caps”, because they’re based on my boundaries. For example, I do not wish to create any more children. That means there’s a cap on how intertwined my life could be with someone who wanted to bear children (with their significant other’s help) could go. Conversely, I’m open to partners who already have children, so there is no cap there. I know a woman who won’t date anyone who has children at all; that automatically puts caps on how “serious” a relationship she can have with someone who has kids.
As with so many other things, this sadly gets glossed over in monogamous dating culture. I think that being forced to think about these aspects of relationships – what you want, what your limits are – can help you and your partner(s) in communicating and strengthen your relationship, whether “serious” or “casual”, “short-term” or “long-term”.
 When I say “ends up”, I have to acknowledge that there aren’t any good, judgment free words here. Everything I can think of implies that a relationship is “better” or “worse” based on where you’re at in that relationship, and that just isn’t so. (Ever start dating someone to find out that you’re better as friends?) I’m still working on phrasing there.
 I’m talking about relationships here, not just sex.