Many projection screens do not provide the angular reflective gain that they claim. This is especially true if you are using a ceiling mounted projector. To see how much gain above or below unity you are getting take a piece of printer paper and either have someone hold it in front of the screen or tape it with painters tape carefully to the screen (this can remove the coating on some screens so do this at your own risk). When you look at the image from your normal seating position if the paper is dimmer than the screen you have more than a unity gain screen. The brighter the screen is relative to the paper the more gain you have. If the paper is brighter than the screen you have less than a unity gain screen.
Comparing your screen material to a sample of Stewart StudioTek 130 can also be useful. This screen material is very close to the published 1.3 screen gain in most installations. Tests conducted in my theater with various screen materials can also be found here.
Be sure to use paper with a high brightness rating for this test. You can calculate the effective screen gain of the paper used by taking the brightness figure and dividing by 100. For example, a paper rated at 94 brightness would be 0.94 gain.
Attempting to evaluate screen materials with normal room lighting is a mistake. You must use the light from a projector to do this properly and view it from your seating position with the projector mounted in its intended position.