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Delta temp is the difference in temp at the heat exchanger. Temp at the register can be influenced by the ambient temp the duct work is in.

So you should check the return air temp in the return plenum or duct at the unit, and the supply temp in the plenum or in the supply duct within 3 foot of the plenum.

On an attic A/C install, its not uncommon for the air temp in the return plenum to be a few degrees warmer then at the return grille. And the same for the supply side. Its very easy to have a true 18° temp delta at the evap coil, but only 14° when measured at the return grille and supply register. On some systems, the duct insulation may be sufficient enough that you won't have a difference, but you won't know that without measuring at the plenums, so it just makes it easier to do all your measure measurements at the unit to check for proper delta.
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