Admob test ads not showing up..?

Hi guys,
Has anyone integrated admob banner or interstitials in their games?
If yes i need a little help from them.
I have done all the setup for the admob banner but how can i know whether the banner is showing or not cause when i run preview it dont show anything(may be thats because they say admob ads will only show in android devices), so i tried to run my game in android but again i saw nothing about my banner ads. I used test ad ID given on google admob site.
Please if someone know how to make sure your banner or interstitial ad are working and showing up without uploading your app on anystore, then let me know

Thank you