It varies depending upon the manufacturer, but with UEFI systems they have removed the prompt to press a key choose a boot device or enter the UEFI (formally known as BIOS) settings. In those situations, the best method is to keep pressing the required key as the system boots. As for what key to use, the best I can really say is to just search for that manufacturer to see what they normally use (some of them can be tricky).
If you want, you could tell me the model specifically and maybe I can help you find it (Time allowing of course).