Text xolor contrast usability testing?

I know sufficient color contrast between text and its background is important for usability, 开发者_Go百科but how can I test for this? Specifically I have tabs and the title of tabs that aren't selected are greyed out but should still be readable.

Currently the site im working on looks fine to me but is there a rule of thumb for people with mild vision impairments (im assuming people with more severe vision impairments would use screen readers, etc).

Thanks

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This toolbar for firefox will give warning & Fails when the color contrast is under a specific amount -

Accessibility Evaluation Toolbar