DS1 isn't deaf either and also has ASD. However, at very much the same age as your DD he was referred for hearing tests and it turned out he had glue ear. In his case it affected his hearing quite badly for a while (and we didn't realise either) but it did clear up before he was 2.
The hearing test consisted of him sitting on my lap opposite a play therapist who was really there to distract him. The audiologist then played sounds at different frequencies and levels which came out of speakers on his left and right side. When he responded to the sound (by turning his head in the direction of the sound) the audiologist activated a mechanical puppet within the speaker as a visual cue to keep him interested and co-operating. By doing this, they could tell what he could hear with each ear.
When he was a bit older, they used another test which put a headphone on the bone behind his ear which conducts the sound waves. Then they played the sounds through the headphone (I couldn't hear them) and watched his reaction. This was to see if the ear was conducting sound properly (it bypasses the ear drum). There are other tests but I think at that age they will have to be done under sedation.
The National Deaf Children's Society (NDCS) is a very good place to start for information - even if your DD turns out not to be deaf, they can provide advice on the types of hearing tests and what they will show (much better than I have done). Their website is here
Paisley is right about signing - I was already doing sing and sign classes with DS so we had a head start on that front. He took ages to use any signs but that was because he has a communication disorder. He understood them though. The chances are that your DD will pick up signing very quickly. There are resources available online and something special is a good programme if you want to learn some signs. I would start with the very basics though, more, milk, drink, eat, all gone and maybe some animals as they are generally the most useful and are easy to pick up.
Hopefully someone who has more experience with hearing loss will come along soon and post some advice.