Sometimes I feel a woman's pussy get noticeably wetter and looser during vigourous fucking.  It seems to occur more frequently during doggie style.

Is she experiencing an orgasm, or is it merely arousal.

I'd appreciate input from the ladies

(Yes, I'm using an alias because i'm a little embarrassed about my lack of knowledge)

B4Slim

when a woman's body is shuddering, and her eyes are rolling back in her head, and she either becomes suddenly quiet or suddenly breaks your eardrums, she's having an orgasm.  Of course, it's not always easy to tell.  Small orgasms can go unnoticed, explosive ones will have the neighbors calling the cops.

maybe you could ask the person you have your dick in that question.
Each woman is a little different as to what she needs for her orgasm.

Women can and do fake orgasms.
If you don't know if she had an orgasm or not, she more than likely didn't.

Personally I tend to be vocal during orgasm, but besides that I will often have a contraction - in my kitty or my whole body if it's very powerful.  
The wetter/looser that you describe occurs as a result of general arousal.  I get really wet when I am turned on.  

Keep going you're almost there.

You're probably mistaking the slipperiness for looseness. Women tend to become inflamed, so to speak, when aroused, and to tense up when orgasming.

Getting wetter and looser are not necessarily indicative of an orgasm, more so arousal.  
I experience orgasms different way but usually

I am sorry it can be so hard to tell, and most times a provider doesn't want to tell a client they didn't orgasm because it can be a disappointment.  
Clit orgasms - shaking legs that are hard to control, arching back, very short but intense. If a man is DATY, I usually have the urge to squeeze his head or pull his hair; most times if there is FIV, I doubt the man is feeling much; I tend to get pretty wet but not nearly as much as I would with a g spot orgasm. The big thing here is, it's not likely I'll have multiple clitoral orgasms because I get very sensitive afterwards. So a lot of times, if we're both doing multiple orgasms, we will do daty, bbbj to completion, a little more DATY until he's back up but before another orgasm and then move into piv
G spot- takes a lot more time and for me is more likely to happen for me in doggy or if we are sitting within each other. Juices are flowing as I get closer; noticeable contractions are the telltale sign that are going to be felt on my male counterpart. If I'm legs in the air I may tremble, but if in mish or doggy probably not much. They're a bit more intense, long lasting, and can roll into multiple easily.

In large part, arousal and even orgasm are reflexive responses.  It is known, for instance, that even rape victims can become lubricated and in up to 20% of cases, orgasm.  This is similar in the way men can be jacked off involuntarily.  

So more vigorous stimulation reflexively (in many cases) will result in more vigorous lubrication.  Happily in a normal situation this also correlates to pleasant physical sensations.

Because the customer before you was a magnum.  It's usually back down to the normal size by the next morning. Be the first customer of the day and you're less likely to experience. Hen you say "loose," do you mean you still had good feeling, or was it like dangling a rope in a well?

Doggie is my favourite position. Although it's also the position that men climax in most frequently

From g-spot orgasms but not from clitoral orgasms. Since it is so wet, it feels looser because there is less friction, not because it is "loose". Doggy is a prime position to hit the g-spot so maybe that is what is happening? The wet from g-spot stimulation is different feeling & tasting than from cock or fingers in pussy stimulation too.  

Personally, I'll take g-spot orgasms over clit orgasms any day. I can fuck for *hours* and not lose my desire/drive or get over-sensitive, which is what happens when I have a clit orgasm.