I've arm checked mine since 1990.   The best way to learn is go somewhere like Graham School down in Garnet, KS.   Well worth it to go there, they bring in uteruses from packing plants so you can see what you are supposed to be feeling.

I've checked for friends too, always took payment in beer.

It's not hard to learn, but you need to be aware that the fetus feels different at different stages, and you may not feel it at all if it's off the pelvic shelf and sitting at the bottom of the cow where you can't reach (usually at about 7 months).   Then you go by tension on the end of cervix and this is where a lot of people screw up and call a bred cow open. 

And, you PG with you NON-DOMINANT hand.    If you are right handed, you check with your left hand.   Seems strange at first, but that's the way it's done.   Don't start out cheating and using the wrong arm, it will only be harder to learn to do it the right way.