The poblano pepper is a mild pepper with a tapered green body. A ridge or seam usually runs along the skin, and they tend to look pinched in the middle. Red poblano are riper and have more heat.
You want them firm and shiny with fresh stems. Wash them well. Put them in a bag in the crisper and they should keep for a week.
Handle them with gloves and wash your hands after. You can make chiles relleno by stuffing them with meat or cheese before grilling. Just cut the stem end off and fill to your delight! Top a steak with grilled or sautéed poblano and onions.