You can consult a site like Linux on laptops to see if anyone has tested a model you are interested in.
If you have a particular Linux distribution in mind, they may have a laptop testing effort. For example, Ubuntu has test reports here.
There are vendors that sell pre-configured laptops with Linux installed. I can't speak to them. I have 2 HP laptops, an 8510p and an 8530p, and they both run Ubuntu Linux well.
It's no longer as much of a hit-and-miss situation as it used to be.
If you have specific brands/models in mind, it's always good to do some online researching, but if you're actually going to a store, I recommend bringing alone one of the Live distros such as Knoppix or Ubuntu. Boot it up on the laptop you're interested in. If it boots clean, check to see if it plays audio, can activate wireless networking (and Bluetooth, if that's important), and make sure it supports the video mode(s) that you want to use. It's probably also worth plugging in an external monitor if you intend to use one.
If you can get a live distro to work well on the computer of your dreams, chances are very good that your favorite flavor of Linux will work as well.
An IDE is no substitute for an Intelligent Developer.