- How long they have been into SEO?
The first and the most important thing you should understand is SEO is not an overnight task and they have to be in it for quite a long period of time for making any success. So ask the same to your SEO provider? How long have you been doing this? Verify the same with their domain who is information and see if they have been doing SEO for so long. - How much they have worked SEOed themselves rather than clients?
The main reason I do not take up SEO job is I am too busy with SEO of my own forum and blog and if I can get my site ranked well I would make more money than any client can pay me. Do they think the same and have their own site or product which they rank very well. - How transparent are they when it comes to working for you?
The next thing you should ask is how they will go about getting good rankings for your site. It may not be exactly what they would do but mainly the strategy. Let them do some keyword research or even go beyond it to analyze your competitors’ site and their strategy. If they talk about too many jargons like PageRank / NoFollow get familiar with them.
To add to this question many SEO experts (as they liked to be called) tend to give that this is something where they have some backdoor entry into Google and can rank you pretty well in Google by paying them and they cannot disclose anything beyond this. This is complete bullshit and you should never even think about replying to such peoples’ emails. Google never wants their users to get results influenced by such sick minded people and never even encourages such people.