Axle bearings: how to tell if replacement needed

Yes, there is nothing holding them against each other right now. If you get them together, then they should rotate silky smooth all around. If you feel any small bumping or irregularities while rotating them together, then replace them. Too much work to get out and set up later to take a chance, IMO.

Get something name brand like Timkens if you can.

If you re-use, clean them in gas or kerosene or something similar, and blow them out or tap them out on something like a piece of wood to get any hardened grease out.

You can use a needle to get the grease into the bearing after cleaning, or use a bearing packer, or hand pack them with the right technique.