In order to work with buyers, I believe that it is wise to decide right upfront that there is a strong possibility that you are not a mind reader and cannot figure out when a buyer is really serious about buyer now, later or never.  Sometimes it appears the person you are talking to, sharing information, driving around, numerous phone calls and emails is just wasting your time.  But you being in the business to make money don't want to assume this person is just gathering information and is not serious about buyer a home now or ever.

You do not want to waste your time or spend time on someone that you could have been spending time with another that was actually serious about buying.

So after you make sure this person is qualified to purchase a home and for how much... I suggest you not assume this person is not serious --> even if they seem to taking way too long to find a home that they are ready to put an offer on.  You must sharpen your patience skills.  Learn how to work with this person as well as do other tasks in order to get other clients - so it will not seem to be a waste of time.

Treat your leads, phone calls, and those that seem to be just site seerers" as read possinbilities. So get your car all gased up, put together the information this client has asked for, study your neighborhood data, put on your smile and get a going....!  You may turn out to be pleasantly surprised!

To the question of how do you know if a buyer is serious?  Answer is:  You don't. Not until the transaction closes.  And always keep in mind... your timeline is not what matters, its that potential buyer's timeline that you should be working with.