To detect unwanted glare, sit at the screen with the VDU off. If you can see lights reflected off the screen, then you've got to remove them. Draw curtains on windows near your computer to limit direct sunlight hitting the screen. Use a side light at night, and lower the general room lighting. Check that the clothing you wear when you work isn't reflected in the screen.
Light clothing reflects more than dark.
When buying a monitor, look for one with anti-reflection coated glass. If necessary, install a glare shield over the screen, and a hood to shield from direct or reflected light. Try to get everything you focus on, at the same distance from your eyes. This reduces tiring large eye movements, and the need to constantly refocus. Touch typing where fingers do the work and wrists remain stationary, reduces eye strain by reducing the wear and tear on the eyes caused by constantly refocussing from keyboard to screen.