Im assuming your question is how do I test a 3.5" HDD to see if it works using a laptop with a slot only large enough for a 2.5" HDD.
on a small number of laptops, the Sata/Power connector is connected via a wire, and can be pulled out of the slot. If you laptop is one like this, then just plug in the hard drive and turn the laptop on (however it will probably not boot up, so boot from a disk or USB).
If the Sata/Power connector is attached directly to the laptops motherboard, then you will need to get some sort of adaptor cable as the hard drive will not fit into the enclosure.
Hopefully this has helped you (as your question was unclear)