The CAA website has a detailed section on the CAA's eye tests for exactly this reason - to save you the £400 if you're likely to fail.
Go to the site:
www.caa.co.uk, medical section, download, print and get your own optician/optometrist (or whatever they're called these days) to check before you go.
From memory of my initial class one three years ago it consists of the following (I'm no expert btw):
Standard distance vision test (read the letters on the wall).
Standard near-vision test (read the text at various distances).
Muscle balance test (where does the line appear on the scale).
Ishihara colour blindness test (hint: the dots form numbers not rude pictures
).
Visual fields test (end of pencil or finger tip initially, with the machine if optometrist not happy)
Intraocular pressure test (air-jet)
good luck.
cheers!
foggy.