1. Ask about their typical successful placement rate in targeted media outlets. Make sure that they arent quoting you random websites that pick up their release just because its on PRNewswire. A lot of websites run a new news section that carries every release that comes across the wire, but Im sure you dont care if a bass fishing website carries a release on your product not exactly your target audience.
2. Ask to see copies of their last three releases. There are a lot of press release distribution websites out there that offer to write a release for you as part of their service. But take a look at their last three issued releases to see how much personal time and attention their clients get. For very cheap distribution services, they probably just plug in the company name and a few key details into a standard release format. As you know, this type of release is very unlikely to get any attention.
3. Ask where theyve been picked up. It can be hard for an untrained eye to look at a release and tell what its chances are of getting picked up for an article, so ask what media outlets have carried stories that their writers have written. This will give you a good sense of the talent youre looking at hiring and will be indicative of your chances of PR success.