My wife and I have been using some small balsa wood markers about half the size of a dime to indicate a "dead" space. A "dead space" is defined as a space that cannot be played with any tile. An example of a dead space would be the tile before or after a hand of five tiles.
Another dead space would be:
tile, tile, tile, space, tile, tile, tile
We use the markers to see where there are possible places to play. The markers also speed up the game by showing areas that are no longer playable, so you don't have to continually rethink those spaces.
We award the person one point for each dead space marker placed. Usually these are placed at the end of counting the hand. However, sometimes, a dead space is found just in looking over the board.
We usually have 30-40 markers down on the board at the end of the game.
You should be able to find those balsa wood pieces at a craft store. The small dots fit nicely in the game board and don't detract from the tiles.
Re: Dead Space Game - Counting the spaces
One question that comes up often in our games is
"when is a space dead?"
Here's some examples.
Using D for dead and B for blank, and assuming these tiles are in the upper left corner.
The B is not dead as a 9 will work. Of course, there may not be a 9 left, but we don't take that into consideration.
In this case, the B is dead as you must have a 3 to complete it horizontally and a 9 vertically.
We usually don't analyze the board too deeply. Usually, just analyze the column and row in question. Here is an example.
The B in (1,2) must stay open as a 2 can fit.
The Bs in column 6 can be resolved with a 5 and a 6, so they are open.
Only a 3 or 9 will work in Row 3, Column 5 - and that won't match with the K5BK combination, so that space is dead. You can only analyze the K5BK combination as 4 tiles as adding a 5th tile would end up with more 6 in a row.
A 3 and 5 wiil work in (5,5) and (5,7) respectively.