how can I detect whether a specific page is mapped in memory?
This doesn't "remap" the memory, but creates another mapping at different address (since the one you give it is already taken, and it's treated as a hint anyway). The old one is still valid, you just lose the reference to it since you overwrite the ptr variable.
If you want to create multiple mappings to the same memory like that look into shm_open(2).
If you just want to check if address is mapped then the MAP_FIXED trick pointed out by @MerickOWA should work.
Edit 0:
You are right about MAP_FIXED, it doesn't help in this case. What you might try is mincore(2). One of the errors it returns is:
ENOMEMaddr to addr + length contained unmapped memory.
On QNX, you can use mem_offset() and posix_mem_offset() and check the contents of contig_len on output, comparing it with the length input parameter.