how can I determine if I have fungus gnats for sure?

Just looking to rule the option out if I have them or not I'm having a magnesium def in week 1 of flower and no matter what I do I can't fix it I've foliar fed 130 ppm solution of epsom salt also added 100 ppm of epsom to last water doesn't seem to be doing the trick I've read that they can cause problems that look like deficiencies that u can't fix I'm in coco I have azamax but I wanna make sure first

you would know if you have gnats, because you can see the tiny pests flying around. just scratch the surface of growing medium n u should see some fly. u can hang some yellow sticky tape/fly strips to see if you have gnats. if not seeing gnats I'd guess your problem is something else. good luck n b happy!

slice up a potato and leave a slice in each pot for 24 hours. then next lights on lift it up and look for larvae on the under side of the slice.

The larvae usually hang out about 2 inches under the top layer of soil. You would see really tiny bugs flying around if they have hatched.

Are these gnats I put soda in 4 bowls under plants this is what I got