You can use the XML editor Shift+Ctrl+X to find out which node comes first in a path.
Try temporarily moving a node, and you will be able to see if the first co-ordinate in the path data changes. Undo the move with Ctrl+Z (Undo).
If you just changed a circle to a path using Object to Path, then assuming you made no further edits such as breaking the closed path at another node, rotating the circle or reversing the path direction, then the first node should be located at the 3 o'clock position on the circle. Similarly, if it's a rectangle converted to paths, then the first node is at the top left. If you use the Bézier tool to create a path or a shape (a closed path), then the first node is the first one you made.
Aligning nodes can be done with the Snapping Controls. Enable Snapping, Snapping to Nodes, and Snapping to Cusp Nodes.
You can also Align nodes by selecting them with the Edit Paths by Nodes tool N. Holding down Shift as you click allows you to make multiple selections. Then you can use the Align and Distribute panel Shift+Ctrl+A.