I have a list
It depends on the needs of the specific situation. For example, the dictionary approach would be quite good assuming:
If the above are not true for your situation, just use
Item wonderIfItsPresent = ... bool containsItem = myList.Any(item => item.UniqueProperty == wonderIfItsPresent.UniqueProperty);'
This will enumerate through the list until it finds a match, or until it reaches the end.