i think if you got a machinist square and indicate one side straight attached to the table and use the other edge of the machinist square to check the other axis this would give you a good idea of the machine squareness. the machinist squares are precision ground and should be pretty close. a 1-2-3- block or a 2-4-6 block used the same way would probably give good results. what you want to look for on the indicator is the amount of taper over a set distance. so if you are checking over say a 6 " length you are not going to want to see very much movement on the indicator. probably .001 or less. but this needs to be based on your actual needs so you could have more tapper if the accuracy is not to demanding. the longer the distance you are able to indicate over is going to give better results so keep this in mind as well.