After five years of hosting I've learned a few red flags to identify potential troublesome guests. The biggest and most common red flag is asking for a discount. I now have a hard and fast rule, ask for a discount and you get politely declined with something like this:
"Hi, thanks for your interest in our place. I don't think we are the best match. I'm sure you will find a nice place that better suits your needs. Have a great trip." When I say "better suits your needs" I mean cheaper.
One of the most surprising and gauling things I've noticed over the years, when I used to offer a discount (when asked), a high percentage of these people then asked for another discount! It usually went something like this:
"Hi, we just love your place but it's over the top of our budget, do you think you could give us a discount?"
I give them a 15% discount
"Thanks you so much. The thing is we're students and we are really neat and clean, could you maybe waive the cleaning fee?"
... sheesh, could I eat one potato chip? The answer to both is no.