how to check if a appointment already made

this is a llittle hard

i have a appointments schedules for the customers that want to cut their hair
now
lets say a customer name john has appointment from 08:00 to 08:30 in date 13-08-2013 with david(HairDresser) witch i scheduled for him
and another customer name edi wants to make at the same time appointment with david and the same date.
how do i make that if david is busy at this time 08:00 -08:30 or on this date 13-08-2013 or edi is already scheduled for another hairdresser
to get a msg david is occupied at this hour or this customer is already scheduled for another hairdresser
this appointments is filled each day with 20 meetings so its a liitle mess to find each meeting with who and when.
anyway
now basicly i need 2 things
to check if the hairdresser is occupied at time from 08:00 to 08:30
and or the customer already has a appointment with another hairdresser
this is the table name and the fields and the values
if any info is needed i will add here
regards salsa31