How can you tell if an SEO service is actually doing the work?

I’m considering getting some help with SEO, but one thing that worries me is paying for a service without really knowing what’s being done each month. I don’t have a massive budget, so I’ve been looking at a cheap SEO service UK options and comparing what different providers include. I’m not expecting miracles, but I’d like to see that proper work is being done and that there’s a clear plan behind it. If you’ve used an SEO service before, what did you ask for to make sure you were getting genuine value?