If you want to force your Android phone to check for updates (ROM updates, not App updates which updates through Market), here is a little trick for you:
First, you could try checking for updates from the *Settings menu -> About -> Updates
*(May vary depending on your model).
If that did not work, you could try this trick:
Open your phones dialer, type ##2432546##
(Or ##checkin## if you like). Just as if you were going to call a number.
Your phone should now check for updates to your ROM.
Note: This does not work on every model though.
Please post in the comments if you succeed!