Some good advice here.
In a nutshell,an older child can usually undertake a standardised test, that is one that allows us to compare him to other children of his age, so that one gets a general idea of where he is in relation to others.
These would be tests like the Reynell Developmental Language Scales Renfrew Action Picture Test LARSP Test for Reception of Grammar, CELF and so on.
One would not generally expect a 2 year old to sit and cooperate(much less understand what was required of him) in such a situation so info must be gathered by other means.
A detailed case history taking note of parent's perceptions is very very useful.
The therapist will indeed watch your child playing, noting aspects such as attention, eye contact, resoponse to instructions/suggestions, willingness to turn take and so on. Also what kind of games they like will be noted.
The WAY they play is massively massively important.
Play is essentially 'work' for children in that it prepares them for using language. It is actually an incredibly complex area where there are many different levels of play. Therapists spend a great deal of time learning about this. There is also another issue namely that of whether they play separately or with someone, whether they are able to play AND listen to and rtespond to someone else (very important step as the child goes from a 2 way interaction, a dyad, involving him and the toy, to a three way interaction, a triad, involving him, the toy and another person.
The most important aspect of play is whether it is symbolic or not (also known as imaginary play)
In other words is the child able to imbue imaginary qualities into a toy/object
Eg pretends a box is a house
combs a doll's hair
feeds a teddy
pretends a stick is a plane
This starts at a basic level (eg feeding teddy) and then becomes highly complex and orderd (eg a 5 year old playing 'shop' with toys, toy shop and maybe other friends.
ABILITY TO USE SYMBOLIC PLAY IS THE SINGLE MOST POSITIVE INDICATOR THAT ALL IS GOING WELL WITH A CHILD'S LANGUAGE DEVELOPMENT ESP. AT THE AGE OF TWO.
This is why a therapist will spend a lot of time encouraging you to play like this with your child if there is evidence of some sort of deay.
As you may know it is often one of the first worrying signs that a child has an ASD (not always, unless anyone reading is panicking!)
There is one very simple lovely little standardised assessment that you CAN administer to a very small child namely the Symbolic Play Test, which is simply a range of toys which are given to the child. As it is a standardised test, we know what children of various ages should be doing with them, so the therapist observes and simply ticks off target behaviours as they occur.
The toys are initially large and simple (eg biggish dolls and brushes, then become more sophisticated, ending with a small detailed scenario involving tractors, a driver, a trailer and a load of wood!
Incidentally some parents don't like this test as one of the scenarios involves a cup and a saucer and a table and a tablecloth. Most people say that as they rarely/never use such things, their child has no idea what to do with them! Don't worry, the therapist will have heard this a million times and it will be taken into account
(no doubt the test will be restandardised in the future because of this!)
Many parents say 'I took my kid to see the salt and all she did was play with him!'
Hope now that you understand why!!
If people asked me what were the most useful things to do with a child with lang. problems (mild) I would say
Turn the tv/video off as much as possible
Sing lots of nursery rhymes to him (brill for language, for many many different reasons)
Read to him a lot and encourage an interest in books
Play in an imaginative way as MUCH as possible
Bin or hide the all-singing all-dancing very sophisticated toys that demand nothing from the child in terms of creativity or imagination
Involve them in a wide range of activities in which you/they will use as wide a variety of language as possible (eg cooking, cleaning the house (!) gardening, painting and so on. Good lang. opportunities are everywhere!!
Hope this has helped.