The accounts command will display a list of all the machines on which you have an account:
% accounts babyruth blowpop charmpop cheeto cheezit chipwich clarkbar crackerjack crunch devildog doodads dorito frito funyon gobstopper goldfish goober goodbar hershey hoho jellybelly jrmint jujube kitkat klondike krackle mandm mentos milkdud milkyway moonpie mounds nachos necco oreo pez poptart porkrind pringle raisinet ringding ruffles runts skittle skor slimjim smarties snickers snocap spree sugardaddy tictac toblerone twinkie twix twizzler utz whoppers zagnut zinger
You can limit the output to those machines which receive mail (and therefore, only those machines whose .forward files you need to be concerned with) with the -m option:
% accounts -m ringding
See the man page for accounts for more details.