I found it a bit like choosing a school. I looked to see what was in my area, checked out their ratings on CQC website and looked on local FB pages for recommendations. I also think that the Home are Association members are meant to meet some standards, so I made sure places were a member of them.
If you Google, there are checklists for what to ask home care providers - Age UK has a guide and Which? might have too. I found that agencies weren't too good at answering the questions I asked though.
There are reviews of agencies, but I wouldn't trust them too much as agencies encourage their clients to rate them and one even paid their staff bonuses if they got a 5star rating from their clients, so they aren't necessarily reviews freely given.
The next stage was that I called a few and they came to have an in home (fairly intrusive) interview with my Mum and I. We got a feel from them at this point and they then handed over their information packs for us to think about in our own time. Although one of them (well known) expected us to sign up without providing their fees!
We then called up our favourite and arranged when they'd start.
Be aware that they mostly have minimum visits a week. One wanted 2x a day every day, others were 2 or 3 hour visits a week. So probably worth asking on first contact.
Good luck - it can be quite stressful, but we ended up with a lovely lady from what seems like a good agency.
Also, beware it's very pricey!! Make sure you apply for Attendance Allowance to help cover the costs.