Starch test is very simple. You can pick up iodine solution at any pharmacy. Place a few drops of clear wort on a white plate or bowl, add a drop or two of the iodine solution, and the wort ill turn blue if there are starches present or yellow if the starches are all converted. Rinse the wort and iodine down the drain and clean the plate/bowl if you are going to use it again.
Since most of the mash is compete by 20 minutes in, you probably have a bit more dextrins that you planned for. Open the recipe and change (edit) the mash profile to give you a mash temperature of about 155F to see where that puts your FG estimate. It won't be dramatic, but it should add a few points to the estimated number.
If you pull another sample, taste it. If it is still sweet, you have a stalled fermentation. If not, then you are most likely done.
If it is a stalled fermentation, I would recommend making a sugar solution (not too much, maybe a 60 grams of sugar [2 oz.]), add a bit of yeast nutrient to it, and transfer your wort onto that into a secondary fermenter. I've found very few other ways of restarting a stuck fermentation (though I have not dealt with it more than a couple of times). It is one of the very few times I have ever used a secondary fermentation.
Let us know how it turns out.