Ask them to brief explain their methodology and most important the KPI's that they will track thru your project.
That way you can compare different agencies KPI's and see the ones that make sense with your business goals.

A few ones to exemplify:
General: Traffic, Sources, Conversion Rate, Bounce Rate.
Adwords: ROI, CPA, CPC e CTR.
SEO: Inbound Keywords, Rankings, External Links (Volume and Quality).
Social: Audience, Reach, Engagment.

and a lot more...

Hope that helps.
If you need more info feel free to give me a call.