When it's being created, a standard 320x50 banner will look much larger on a desktop monitor with a larger pixel size than a mobile device with much smaller pixels...and each handset has a different pixel density and hence size on screen.
I suspect that a lot of text heavy banners get shrunk into illegibility because of this!
How do you check that your banners remain readable on mobile devices?
Mobile emulators?
Go to browserstack.com and check your ads or landers
Yep check it out on some sort of emulator if you want.
Opera mobile emulator or chrome dev tools come to mind.
I like real phones, then browserstack then free emulators in that order but I'm someone of a LP testing snob....90% of the time I only test on real phones.
The increased pixel density of mobile phone screens actually means it's easier to read smaller text.
The high pixel density means the fonts look smoother and more legible at small sizes vs the same physical size on a desktop monitor.
You would be surprised how small the fonts can get before they are hard to read.
As a test, try downloading a high-res picture of the iPhone 6 home screen. Now find out how wide the iPhone 6 screen is (say, 3 inches).
Now measure your widescreen monitor width and divide the two. Say you get 7... now open up a Photoshop document at your native resolution and tile the iPhone 6 homescreen image across it and view it at 100% zoom in full screen.
Chances are it is no way near as sharp, crisp and readable as the home screen on the real thing :-)
Example 1920x1080 image:
