[SIZE=14pt]You probably wont ever be able to hear the foals heart beat even a trained ear with a stethiscope, I could never hear them..... but you can start to feel movement about 6-7 months if you wrap your arms around their middle and feel gently just in front of the udder usually when they are eating the babies are more active.[/SIZE]
I agree with Lynn, about 200 days if you are patient, you may be able to feel a kick and it might feel more like a flutter but by the end of 7 months you should be able to feel it.
I always start trying about 190 days because I get impatient but it is more towards the end of the 7th month that I have had the best success.
And like Lynn said, no heartbeat, never have heard one.
To date I have detected movement in 11 of 12 mares at 5 1/2 mo's. I may check a few times a day and my arm gets tired and patience wears out after 15 min. Hardest part is concentrating on your "sense of feel" when checking as it is easy to daydream about that filly in there and then you can miss the subtle activity at the early stages.