Correct way to measure a wall wart voltage
It sounds as though you've got some unregulated power supplies. Generally these have a transformer, rectifier and smoothing capacitor. With no load the capacitor will charge up to peak voltage.







simulate this circuit &ndash; Schematic created using CircuitLab

There is always some internal resistance in the transformer, represented by R1, which will cause a voltage drop which increases with current drawn. In addition, C1, which maintains output voltage during troughs in the rectified AC (i.e., between pulses from the rectifier) will droop between cycles causing the average DC value to droop with increasing current.

To test you should load them with something simulating the intended load. e.g., for 100 mA load on a 9 V PSU you would use a 9 / 0.1 = 90 Ω test load. Nearest standard values are 82 Ω or 100 Ω. Connect the test load across the terminals / connector and measure the voltage.

An alternative is a 12 V, 6 W car tail-light lamp. At 12 V this should pull about 0.5 A. Be aware that the resistance value will change dramatically with temperature which is voltage dependent. To get a good feel for what's happening you would need to measure both the voltage and the current.