First let me congratulate YOU for asking a great question. Most of what I read here are stupid questions and even dumber answers. Yours is a very good question.

Most people wouldn’t even know to ask it.

Here are just a few things you should ask an SEO “expert”

What SEO tools do they use ? example – for keyword research they should mention at least one of these Wordtracker; keyword discovery; or Google keyword tool.

What’s the difference between PageRank & ToolBar PageRank ?
If they say “they’re the same” end the interview. The first is daily the other quarterly.

Which SEO’s do they look up to ? Look up those names to see if those people know what they’re doing.

What has been their biggest failure on a project ? If they tell you they’ve never failed, this is either their first project or they are flat out lying.

There are more but you get the idea, don’t forget to Google their name to see what if anything has been written about them, and if you can’t find anything ask them “Why not.”

Good luck because there are some great SEO’s out there floating in a sea of bullshit.

Let me think, maybe below question can help you
1. How to increase your SERP
2. How to increase PR
3. Did nofollow link help on SEO