Best way to check if object exists in Entity Framework

When using Entity Framework, the best way to check if an object exists in the database from a performance point of view is to use Any(). This is because Any() will return as soon as it finds a match. Another option is Count(), but this might need to check every row before returning. Here's an example of how to use it:
if (_context.Customer.Any(o => o.Id == _idToCheck))
    // Match!
bool clientUsernameExists = _sb.Any(x => x.Username == _username);

if (clientUsernameExists)
    throw Exception("The client username already exists");