I have just downloaded something from Qld Health, its a checklist that Health Professionals use/hand out...
Under the Talking and Understanding section (for 2 year olds) it says they should be able to:

- say 20-50 clear words (he can say just over 20 words, but a lot of them are not clear/perfect)
- say clear two-word sentences (has only done this once that I have heard?)
- name pictures and objects when asked (yes he does this, but only for the objects which he can say the names of obviously)
- follow two step commands (yes)
- show a broad understanding (receptive vocabulary) or 50 or more objects and pictures. (definately yes)
- show interest in books and stories. (definately yes)

So yeah, he is definately a bit behind on the speech part, but I do know he has no problem with comprehension... and he understands a HEAP.
for eg. He has a series of books all about different types of vehicles, - 80 vehicles in total, and he knows what every single one is called... if we try to trip him up by pointing to a front loader and telling him its an excavator he will get very irate and knows thats not what it is, and this applies to all pictures/objects... so I do know he can understand language perfectly...