Determine the time shift for your time zone relative to Greenwich Meridian. It can be recognized, for example, from the settings the system time in the operating system of your computer. Click the clock in the notification area of the taskbar with the left mouse button, and the screen will pop up an additional socket with calendar and analog clock. In its lower part is placed a link "Change date and time" - click on it. The system opens additional settings window with three tabs, one of which is called "date and time" and has a section "Time zone". In this section you will see a shift of your local time relative to Greenwich mean - corresponding label may be, for example, "UTC +04:00 Volgograd, Moscow, Saint-Petersburg". This means that the system clocks of your operating system is shifted by four hours forward with respect to the zero time zone.
Take the time shift set for your time zone from the current time to determine the corresponding Greenwich mean time. For example, if your clock shows 26 minutes past eight in the morning, and the time zone is correct Moscow time, the shift is four hours, so GMT you have now 26 minutes past four in the morning.
Use the online services to determine Greenwich time, if you have Internet access. This is an even easier way. For example, you can click on the link http://time100.ru/gmt.html and the current GMT time see immediately, without any additional calculations.