Thanks laskern.
However, a shuruk, also known as a kubutz (the three diagonal dots underneath) can sometimes be a tenuoh gedoloh as well, in lieu of the melopum, as in the word mezuzos in the shema. The clue is that it is an unstressed open syllable.
A chirik without a yod can also sometimes be a tenuoh gedoloh. This occurs for example when the word “im” in a posuk has a cantillation mark, as opposed to being hyphenated (makaf). This is similar to the difference between “es” and “eis”. For example in hallel “im nedivim im nedivei amo” – the first im is hyphenated and is a tenuoh ketanoh, while the second is accented and is a tenuoh gedoloh, and should properly be pronounced “eem” – slightly longer than “im”.