Hope this might help...
For three dots in a row:⌥;
The result: …
(⌥ is the symbol for the Option key)
For a checkmark:✓
Open System Preferences > International > Input Menu. Place a check next to "Show input menu in the menu bar." Now your country flag should appear in the menu bar. Click that and select "Show Character Palette". At the bottom, search for "Check Mark". You can then select your favorite check mark, and you can add it to your favorites for quicker access via the character palette by clicking the gear icon in the bottom-left and choosing Add to Favorites.
Also notice the Show Keyboard Viewer option when you click on the flag. That brings up a little virtual keyboard that shows you what characters will be entered when you click that key. Bring that up and hold down the ⌥ key, or ⌥⇧ (⇧ = Shift). Notice how the keys change into symbols. This way you can get a quick glance at how to enter a certain symbol.
For further reference, here is a list of shortcuts for various symbols:
http://usingmac.com/2007/12/12/100-mac-k...
And for even further reference, here's what various symbols mean in the event that you run into them:
Apple (easily entered with ⌥⇧K; the Apple Key is the original [and today, less common] name for the Command Key)
⌘ Command
⌃ Control
⇥ Tab
⇧ Shift
⎋ Escape
⌥ Option
⌦ Delete (or fn+Delete on a laptop)
⏎ Return (also ↩)
⏏ Eject
⎌ Undo
⌅ Enter (also ⎆)
⇪ Caps Lock