Am not a SALT, but have been through this with dd1.
Dd1's first assessment was at 2 ish (can't remember ) and she had very limited language at that point - mostly only understandable by me.
The appt took about an hour, with a big long chat with the SALT - taking history etc - what she was interested in, did she reeach for stuff/point/communicate needs or wants in any way. whether she played appropriately, and whether I thought she understood what I said to her/asked her to do.
The SALT had a few toys out for dd1 to play with, and a few books to go through with her, and just approached her as an adult trying to get to know a toddler would - tried to get ehr looking at books together, watched what she did with the toys, and either joined in or asked her things about what she was doing (push the red car, show me the horse, etc)
dd1 was very delayed with language (she is ASD, and her biggest problem at 4 is still language delay), and was pretty non-communicative at that age so maybe with a more interactive child things would be different, but thought I'd add my experience.