Is my psu local and bad

tomorrow i bought a cooler master 400w psu but it didn't came in a box , the seller told me that it doesn't come with a box but he handed me a warrranty card for 2 years on this psu , it has 17 amp on 12v rails and i had seen pictures of coller master 400w model on internet and it is the same that i have but how can i know that he didn't cheated me witha cheap 200w psu with a sticker of 400w of cooler master please help me .
my pc specs
intel core 2 duo e7500 @ 2.93 ghz
ram-2gb
hdd-320gb
gpu-sapphire radeon hd 6670 1gb ddr5
mobo- as3u3s3 p5kpl am in
and if its the real cm psu will it be sufficient for my pc .