I have been seeing posts with levels and straight edges and that can all work but just make it simple. When I ran the swap meets I just carried a piece of string like an Egyptian building a pyramid. A string pulled tight is a straight line. If you put the string on the #1 port edge inside the cut edge then align it with the back port edge on the same machined hole you will see if the manifold ports are in alignment.
Manifolds move up and down in the center without proper glands and rings. The manifold moves and not the block so don't worry about in and out, just up and down. Sideways doesn't exist as the car won't run with leaks and clamps will break it or it will run like crap. In my first college class a professor asked "how do you make a straight line?" I answered "pull a string". He asked if I wanted to be an engineer and I told him no I have no interest in trains.
The best way to check for straight is make four discs that fit the exhaust ports in the block------tack weld a length of 1/2 pipe using a block with the discs in it to hold the discs in place and make the pipe long enough to over hang the block at each end---the overhang gives you handles to pull the hot alignment jig from a manifold bender checking and knowing where to start bending BEFORE you heat the manifold-----starting in the wrong spot can ruin the manifold by spreading the wrong ports. No tape, no straight edge--- no hot fingers!
Spoke to a buddy about this recently. He had some bent manifolds for sale, (cheap), at a swap meet some time back. Perspective buyers would hold the manifolds along the edge of his vendor table to check the hole alignment. Yup, they're bent. My buddy swore that for the next swap, he was going to trim the edge of the table to a slight arc. Nice & straight manifolds!!!
(Yes, it was a joke)
You can even sight along the edge of the manifold to see if they are straight.
I keep a measuring tape clipped to a front pocket because sometimes I need to use one and don't want to go looking for it. Less likely to get lost than a piece of string.