Even once you know who to avoid, then you should still ask many questions about any policies that you are considering.
The most important is 'lifetime or annual'? Annual policies are cheaper, but if your pet is one of the 20% that develops a chronic condition, then you'll be left to foot what could be a very expensive bill yourself after the year has passed. If you can afford it, then go for a lifetime policy, especially for dogs.
Another important question is 'How much insurance do you need?'. As a rule of thumb, a cat should be covered for at least £4K per condition, over its lifetime, and a dog for £6K. This should cover the cost of most diagnostic, surgical and medical procedures but if your pet has an especially rare, complicated or serious condition then costs may exceed this. Dogs are not just big cats, they are generally more expensive to treat as a species. But if you have a chihuahua things are going to be cheaper than if your pooch is a great dane, so you can probably take a gamble on less cover. If you are going to take out a cheaper policy, then make sure you are able to top up any costs if your animal requires referral to a specialist centre.
You can find a good checklist of other questions to consider here . And even though your vet isn't allowed to recommend a specific company, if you question the staff about the policies you are contemplating, you are likely to get an idea of whether they have a reputation of being a good payer or not.
Photo by Flickr user Turtlemom4bacon