How do you know what the talismans and tools in the shop do without having to buy them first?

In the shop you can tap on the picture of the item to get a description of the use.

For more questions for Sherlock・Hidden Object Mystery check out the answers page where you can search or ask your own question.

Go to "Shop" which is cash register with handle on bottom center of main screen. Click. then at the top are different options like Talisman and other items on tabs. Click on the photo area of an object and it explains what it does in a pop up box. You don't have to use or buy it to escape.