One kind of centre finder on the market has a slot for a pencil to mark intersecting diagonals. These work well on squared timber, but are a bit slow to use on a batch of items. Another kind has a blade. You position the blank, tap it so the blade makes a diagonal line, then repeat. I’ve found that this type can easily start splits in the wood, and needs the end of the blank cut square.
The centre finder I used for a long time is simply a small wooden block. It has a hole drilled that is a push fit for a short bit of pencil. The hole is off centre. By turning the block on different sides there is some height adjustment. To use it, I put the spindle blank and the pencil block on a flat surface and mark a line across the end of the blank. Then I flip the blank 90 degrees and repeat to do all four sides. The result is a small square marked on the end of the blank, from which I can easily find the centre by eye. This is not super-accurate, but good enough for most purposes, and it is quick. I have several blocks in different sizes so I can pick the one most suited to the size of the spindle blank. The blank need not be square and the pencil block can mark cylinders equally well. The point of a nail works too, and stays sharp.
I’ve now made a deluxe adjustable version of the pencil block, with a pivoting arm to hold the pencil. The hole for the pencil has a slot through it and a pinch bolt to lock it.
When part of the spindle is to stay square after turning, I need a more exact centre For this I made a cylindrical insert for a four jaw chuck. I use engineering jaws in my chuck, but any self-centring chuck should work. The insert is a snug fit in the body of the chuck and has a steel point inserted. The centre mark left in the insert after turning it to size gives the place for the point. To use this centre finder, I put the chuck and insert on the bench and adjust the jaws to a loose fit on the blank. Then I insert the end of the blank, twist it slightly to align it, and give it a tap. The steel point marks the exact centre nicely. A batch of same-size blanks are done in no time. The blank must be square to use this method.
To find the centre of a disc or square, I usually use a hardboard disc of proper size. I have a lot of these in different sizes for marking out bowl blanks, and each has a small hole in the middle. I just lay one on the blank, and place it quite accurately by eye or touch. Then I mark the centre with a pencil through the hole.