I'd suggest that on the first draft you don't stress about it. Concentrate on getting the story out. Go back and weed out the tense issues after.

Then try reading the stuff out loud. Tense changes are easier to spot when you're speaking than reading; I have no idea why.

I'm told some grammar checkers spot this stuff too, but I'm not sure which.

If all else fails, do a final edit using a Search for present tense keywords like "goes", "says", and "thinks".

That's the best I can offer.