how do you know if patent is design or utility, as no marking is mentioned on the patent?
There are a few ways.
For one, US design patents have a USD number, for example USD543181:
In google it also says "US Design Grant".
The USPTO page is a little harder, though
CLAIM The ornamental design for a folder-type cellular phone with digital camera, as shown and described.
in combination with only figures and no description is a good hint.
Or if you look at the pdf, it says "United States Design Patent".
While your example does not say so and that's how you know it's a "normal" patent.