Wash the chile pepper first. 

There is no certain way to tell if a pepper is hot by looking at it, although I have heard many, many wife's tales about dark spots, cracks and anything else. But the only sure way to tell is by tasting. 

Slice the chile pepper down the center with the tip of your knife.

You can see the membranes and seeds here in these two halves.  The membranes are where the capsaicin is stored.  It is this part of the chile pepper that carries the heat. 

The seeds are not as hot, but since they are a part of the membrane they do have a slight bit more heat than the green outer chile pepper. 

If you are simply dicing your chiles, the easiest way to clean them is to cut them into quarters. Then you can simply slide a sharp knife carefully down the green outer part of the chile pepper removing the white membrane and the seeds. 

If you are making a dish which requires chile halves, you can still clean them out easily with a sharp knife run along the inside edge to cut loose the membranes and remove the seeds.  Just be very careful, as much as a small cut from a sharp knife hurts, it is much worse when it is touched by chile pepper!

How To Prepare and Roast Fresh Poblano Chiles:

Fresh Poblano Chile Peppers
 

Roast the Poblano chiles either in a cast-iron skillet over medium high heat, under the broiler, or over an open flame (such as your grill). Turn the chile frequently to prevent over cooking on any one side, and let the entire chile pepper become charred and blackened. 
 

Remove from the heat and place in a slightly damp towel, allow to cool. Cooling the chile in a damp towel will help to loosen any skin that is still stuck on the chile pepper. 

Once cool, carefully peel the skin from the chile pepper, remove the stem end, and slice open to remove the seeds.
Once this is done, dice the chile in 1/2-inch by 1/2-inch pieces; set to the side until ready to use.

Hatch Chile Peppers - Blistering and Processing Hatch Chile Peppers for Freezing
 

With a damp cloth or under running cool water remove the dust and dirt from the dried chile peppers. To remove the seeds from a dried chiles, pull or cut off the stem, split the chile pepper lengthwise, then scrape out the seeds with a spoon.

In a large pot, place chile peppers and cover with hot (not boiling) water and let soak for approximately 1 hour (don't soak more than an hour and save the juice to spice soups).

NOTE: Some people remove the seeds and membrane after the soak process.