as long as you wont change the scoreboard files or do an:
scoreboard players reset * foo .....or something its saved in the scoreboard files and stays there, even after an reload.
to check an score use:
/testfor @a[score_foo_min=3000,score_foo=3000]
this will give you an output if one player on your server has an score of exactly 3000
so its between min and max, in this case 3000