Down syndrome can be diagnosed with a blood test but as balham mummy5 said usually it is diagnosed at birth or in the new few days. I have heard of families where the diagnosis took weeks but I think it is rare for a child not to be diagnosed at 1 year, although not impossible, if a child has missed the early checks. In my experience health visitors don't always chase up families who do not attend health checks.
There is also Mosaic Down Syndrome where the condition is often less severe and the child can look less like a typical child with DS, and again a blood test can usually diagnose this.
In a way the diagnosis is not really that important, what is, is how the child is supported. Most children with DS will have some degree of learning difficulty, they will usually meet developmental milestones later than their normally developing peers and will require a degree of support from physiotherapists, speech and language therapist etc... Lots of areas have portage services which provide pre-school educational support for children with DS, and this support can start before a child is 1 year old.
Does the child have developmental delays? sometimes this might be the way to get the parents to start thinking about how best to support the child and not to worry if it is DS or something else.
The Down Syndrome Educational Trust and the Down Syndrome Association both have execellent websites with lots of advice and support if you need them.
I hope this helps.