"对着光检查" can be translated as "inspect against the light" or "check by holding it up to the light".
For example:
You should inspect the document against the light to see if there's any watermark. （你应该对着光检查这份文件，看看是否有水印。）
Let me check this stamp by holding it up to the light. （让我对着光检查一下这张邮票。）