Most plastics are marked, permitting easy identification. A small triangle with a numeral inside appears on the bottom or other inconspicuous part of the container. If a 1 or 2 is inside the triangle, it is recyclable.
For consumers, single stream recycling is a hedge bet. There are ways to make your recycling more green and more economical. Avoid joining mailing lists and switch to email and electronic statements. Print on both sides of the paper to reduce the amount of paper that you use. Consider whether you need to print at all: apply for a job online or submit a resume by email instead of printing it. Maybe you can copy and paste information and email it to yourself, apply labels, and maintain a virtual filing cabinet. Buy products composed of postconsumer recycled content (which may be even more important than whether you recycle your double-sided paper.)
There's hope that single stream recycling will greatly improve