If you want to check the composition, temporarily switch the mode dial to the P, A, or S mode. Afterwards, check the composition and then switch back to M mode to shoot the picture.
Alternatively, simply lifting the built-in, internal flash of the camera allows the Live View image to always display with the appropriate brightness to check composition - providing nothing is mounted to the shoe mount preventing the flash from being raised. To reduce the influence of the built-in flash, however, make sure to reduce the exposure value (EV) after raising the internal flash or use a diffuser.
NOTE: Additional information on using studio flashes is available in Knowledge Base Article C538252 .