When observing canned peaches, the first sign of well-canned peaches is its color. It should be golden yellow and a little blueish. When the color is extremely yellow and uniform, it is most probably fake. As for the syrup, in natural peach, the syrup would be colorless while with dyed peaches, the syrup will be yellowish.
For canned strawberries, the most important thing to pay attention to is its syrup. If its extremely red, preservatives and food coloring might be added to it. Slightly pink syrup would be acceptable as the juice from strawberries might be mixed in with the syrup. Its flesh should be slightly crunchy since they didn't reach ripeness completely.