First I ask the inquiring potential buyer if they have spoken to a lender to find out what price range they should be looking in. I always let them know it is to make sure I am not wasting THEIR time showing them homes they may not be able to afford. Should I show them $100,000 range? $200,000? $50,000? If they have not spoken with a lender and don't know of any to call, I refer them to a trusted local lender who will do a quick pre-qual over the phone. He knows the proper questions to ask. He then lets me know if they even qualify and what price range based on his expertise. Then I sit with them to find out ther needs and wants and in what area. At that point I know if I can show them the house they had called me about originally, or if I need to search MLS for others. After they have seen the good, the bad and the ugly, I make sure they know it's time to move forward with an actual pre-approval so they will be ready to make an offer when the time comes. If they hesitate to take that step, I let them go because they have shown themselves to not be serious buyers.
If, upon that first contact, they tell me they are pre-approved, I ask for a copy of the lender letter, and explain why it's important. If they never supply it and/or I never hear from them again I know I didn't waste my time.
All that is, of course, after I have also queried them about whether or not they are already working with another agent. If they are and have no problems with him/her, I urge them to be loyal to that agent because we are only paid when an escrow closes, not when we show a house (some buyers think agents are on the office payroll). Some buyers have told me that their agent is busy and had told them to contact the listing agent to see the property - in which case I tell them to have their agent contact me. [ I have had cases where buyers told me areas they absolutely did not want to consider; I showed them elsewhere for days - sometimes into the next county; they stopped answering my phone messages; they bought a house in the area they told me they didn't want - obviously using different agents for different areas ]
So I do try to make sure they are serious potential buyers in a realistic price range before I commit myself.