What is the fastest way to check the functionality of a smartphone and notebook?

I have in front of me to buy a new smartphone and notebook this year and before buying I would like to check their function because someone else has already claimed this year that his item was working and it was broken when I tried to use it. What is the fastest and safest way to do this (so as not to waste time)? Who knows what to do?

If so, I order something through a portal like Ebay or http://www.notebooks-billiger.de.

Here you would have at least the opportunity to test briefly and the somewhat higher "security" to open a case with PayPal purchases there - if there are legitimate problems.