I have a ClientCollection and I'd like to check if a certain Client entity with clientid='123' exists in the collection? I don't actually need the entity, just if it exists or not. (If it returns it, that's ok, too.) Is FindMatches the correct method?

using llblgen 2.6 VS 2008

Yes, FindMatches is the way to go:

IPredicate filter = (CustomerFields.CustomerId == "ALFKI");
ArrayList indexes = myCustomers.FindMatches(filter);
// found
if (indexes.Count >0)
{}

You also could use Linq2Objects:

if (myCustomers.Where(c=> c.CustomerId = "ALFKI").Count()  > 0 )
....

You also can use Filtered EntityViews