It’s taken me a long time to settle on a method for punctuating thoughts. A lot of the older children’s novels I read (from the 1950s and 1960s) punctuate thoughts with double or single inverted commas, but this makes the thoughts almost indistinguishable from the spoken words.
Eventually, I settled on using italics to differentiate thoughts:
I shut the window carefully and stood there. I should go back, or at least check that I can get back, I thought.
I went online to check the integrity of this solution and was quite surprised to see that italics is the solution favoured by most writers these days. Well, that’s one thing I won’t have to change drastically.