I have 4 dogs so when we hit the off leash park it would be easy to lose track of them all... however I taught all of my dogs to watch for me too. This means that as we walk they have learned to naturally check where I am and stay close (close has a different meaning for each of the dogs... Russ thinks it means within shouting distance while the others hang out a bit more in eye sight).
Teaching your dog to check in is easy! Once you've mastered a good recall and are ready to be off leash safely you can begin working on this one. Bring some great treats with you and randomly reward your dog for coming back to you on his/her own (you didn't call but they just came back) or making eye contact with you as you walk along.
Over time your dog will begin to check in because it's been reinforced. This means that your dog has decided that you have value and he/she might be rewarded for paying attention rather than being lost in the dog heaven that is the off leash park.
This is particularly important for independent breeds such as your terriers, northern breeds and many rescue dogs. While I don't continue to feed my dogs forever at the park I still continue the practice once in awhile. It's important (especially with a large group) to have dogs that care about your location.
I have found this skill to be extremely useful when we're hiking out in the mountains or walking in a very distracting area. The check in is easy to teach and a must have for off leash walking!