How can you tell....

[Emac]

I think there is a danger in making assumptions. As a relative new comer to some of the modelling sites, I find there is an unfortunate snobbery among those who have supposedly 'made it' and thus consider themselves to be in a certain bracket. They disparagingly use terms like 'GWC' (guy with camera). The reality may in fact be quite different (which is not to say there aren't time wasters). But there will be people who are comparatively new to this thing, and it may well be that they need to ask a few more questions and seek more clarification than others may.
You could easily dismiss someone who - far from not being serious - is simply finding their feet but never-the-less takes what they do serious and strive to become more experienced and professional. I'm sure everyone started somewhere.

[ModelsFirst Paul]

I do have to add:

Whilst most are okay, there are some models are WAY too quick to assume the worst. It's happened a few times that when I have contacted a model about a shoot, with only the essential info to start off with and my main question being the model's availability - she replies stating that I am a timewaster for basically not giving her a full A-Z of the shoot and a full DNA analysis of all her co-stars!

There are many, many different shoots that I have to cast models in, and unfortunately I can't spend 20 minutes on each email giving details to the n-th degree.
So whilst I agree that email tennis is definitely annoying, I prefer to give only the essential info in the beginning, establish availability, and then give further complete details via phonecall. One last email then follows with confirmation details and a quick, basic re-cap.

So basically, what I want to say is that you must trust your instincts and common sense. Rather than have a blanket approach to all incoming enquiries, take each one on its own merits.
Also, find ways to make life easier for yourself. If you can't research the person/company and/or their work, and they're also contacting you from a free email address etc - then spend less time on that front. The more time you spend on someone your gut is saying is a timewaster, the less time you have to spend on worthwhile pursuits.