Most (pretty much all?) rental aircraft will not have an MEL. Although I have no personal experience getting an MEL for an aircraft, I understand it's a rather involved process (involves FSDO, etc.) and must be done specific to the aircraft registration (e.g. a flight school can't just get one MEL for all of their 2003 C172 SPs, each one has to have its own specific MEL).
For most of the Part 91 rental type stuff, 91.213 applies. Basically you have to have the equipment required for VFR Day and or Night/IMC ops. Plus all of the equipment listed as required on the aircrafts equipment list (usually stuff like stall horns, etc.).
The inoperative equipment also has to be deactivated and placarded as "inoperative".
So "TOMATO FLAMES" + Req'd equipment on the master list (for day VFR anyway)!