From my experience, Joey, auditing is the key to monitoring SLs.
I used SLs in what was, at that time, a fairly unconventional application. Workforce varied between 12-15 staff, handling all ranges of customers (inquiring, new, some experience, lots of experience. Then large, small, global, nation-wide, for and not-for profits...you name it).
I would audit accounts: Are they receiving what they are supposed to? Is the pricing right? Is the timing right? Are the numbers right? Are they hitting their service level numbers to gain the most service with the best pricing?
I would audit workers: Are they providing the service required, by service level and by customer? Are they invoicing correctly? Are they positioning services correctly? Are they providing a higher service level than the contract specifies? Are they providing a lower service level than the contract specifies?
Then I would survey. I would call accounts/customers and ask them the same questions that I asked when doing my internal auditing. I would talk to workers and ask them the same questions as well.
Hope this helps!