How can you tell if

a spark plug is cross threaded?

If you take it out it should be able to be taken out with your fingers after getting it loose, Look at the the threads on the plug after it is out and see if they are 'messed up'. They should be clean and smooth, compare it to a new plug to see.

They sell rethreaders for this usually two different sizes with a space in the middle for your spark plug socket. Coat it with greese before using it I use them occasionally on plugs that are rusty. Use anti seeze on the new plug threads. If a plug doesn't start easily I would suggest using one. Here is a picture of one most parts stores carry them.
http://www.autopart.com/tools/toolsmain/tool/T_2360.htm

They sell rethreaders for this usually two different sizes with a space in the middle for your spark plug socket. Coat it with greese before using it I use them occasionally on plugs that are rusty. Use anti seeze on the new plug threads. If a plug doesn't start easily I would suggest using one. Here is a picture of one most parts stores carry them.
http://www.autopart.com/tools/toolsmain/tool/T_2360.htm

Thanks! The truck is almost apart, we decided to fix it the right way for piece of mind since my daughter is driving it.

Since I didn't look at it I can't say but I have used this tool and did 3 more plug changes without a problem. With new iridium 100,000 plugs it should last the life of a used vehicle. Stripped out plug holes are another problem.