this is the worst nightmare for any developer! We have all been there.
YOu need to find out exactly what browser and what version your client is using - some old browsers do not render CSS the same
Next find out what resolution they are viewing at and at what colour depth.
I design for 800*600 px in the UK because thats the most popular resolution but I am moving towards 1024* 768
Then test using their settings. Make sure they are not using AOL because AOL can render in weird ands wonderful ways.
If that doesn't profer a solution then do one oer all of the of the following:
1. Take a laptop on site ans show them it's working OK
2. Tell them to look at it from home
3. Tell to look at it from a PC shop
4. Ask themto get their friends to check it.
5. As a last resort post a link on a forum and ask peopel to check it for you and post their specification.
If 20 people respond you at least have 20 posts to forward to the client.
It is a pain I normaly find it's because thie resolution is to low. Remember all websites look different at various resolutions.