it is easy to check your film holders with a caliper or by shooting a sequence on a flat wall.
Here are some other things to pay attention to:
is the film loaded correctly in the grooves?
Did the focus move when locking down the standards?
Did the focus move when inserting the film holder?
Is the film holder inserted completely?
Did you move anything when cocking/adjusting the shutter?
Does the focus shift (optically) when stopping down your lens?
Is the tripod sturdy and completely locked?
Are there vibrations during exposure even when locked down?
Is there subject movement? (trees are always moving even when they dont look it)
*it is probably hard to remember now what exactly happened.
Pay attention to these. Make notes of your exposures and which holders you use.