How can I test whether two disks have the same geometry or not? I want to use dd to copy the contents of one hard drive to another, but I don't know whether they have the same geometry.

If they have the same blocksize (512 byte per sector is common for harddisks) there should be no problem.
Blocks are numbered from 0 upwards, search for LBA (logical block addressing) scheme.

dd if=/dev/hda of=/dev/hdb