I know where you are coming from, as I also get a lot of people contacting me for projects that don’t happen or clients who aren’t serious, don’t have budgets, etc…
but,
I think for a small consultancy or freelance, making prospective clients answer a questionnaire sends the wrong message. essentially at first contact you are presenting a barrier to working with you and maybe scaring them off. validating clients is important, but it’s equally important that you don’t present a feeling of “I don’t want to work with you, unless you jump through hoops… My time is too important to waste on getting to know you”. If I were looking for a designer, even knowing the business and someone presented me with such a thing, I’d be immediately turned off and would go elsewhere.
What’s better, and what I try to do is ask a few more general questions in an email to get and idea of where they are and what they want (esp. if the initial contact email is very brief), and then judge them by their response. Similar idea to the questionnaire, but not as formal and can be more personalized depending on their initial contact.
typical questions you might want to ask is-
- what’s your deisgn budget?
- do they have the capital expenses in place for development/production
- what is the timeline
- what are the projected production quantities
After that a good idea would be to have a brief phone call (you can have them call you to save LD calling costs or arrange a skype call), to chat with them and get more details. guaranteed in 5 minutes of chatting, you’ll have a good idea if they know what they are talking about or are a waste of time.
I’d also recommend it’s a good idea to have a consultancy mailer or web site of some sort that gives an overview of your services and capabilities which you can direct them to to give more info and possibly answer questions they may have.
one other bit of advice i’ve discovered to get rid of those potential clients that keep emailing with questions, back and forth useless contact, and such before any proposal or real contact is to email something along the lines of “sure we can do that type of design work. do you have the $200,000+ capital needed for tooling available?”… most often you’ll never hear back if they aren’t serious or have no idea what is required to bring their “idea” to market 
R