I have a rifle that I am almost done with last step is to put on the geissele free float rail I was wondering what is a good way to line up the rail and the receiver can I use my aimpoint t1 mount or magpul pro rear sight? I have been told to use a one piece scope mount meant for a long scope however the only ones I have are the 2 piece ones and I do not want to spend 70 dollars on a new mount. Thank you.

The human eye is a remarkable organ.

Upside down on a flat surface

Detachable Cary handle also works.

All great suggestions of using visual alignment, carry handle.

If you ever think you're going to build another one, I recommend picking up the YHM picatinny vise attachment.  It is very good for these purposes and about $70.

Upside down on a flat surface works well.  If you want to be more exact and hold the pieces together while tightening everything, look for a cheap 1/2" high 4" long Picatinny riser.  Lots of companies make them.  Chinese is good enough for this limited purpose - should be under $20.  Bridge the riser between the rail and the upper, bolt everything together and then remove the riser.  Save it for next time.  There will always be a next time, even if you don't get a carry handle or a one piece mount.

While I agree about the YHM, a carry handle can be picked up for 50$ plus or minus 10 on the EE. Carry  handles do a good enough job, and can be used as a Iron sight

Eyeball is always best.