Note the color. Typically, Ports start out very dark in color and then that color fades with time. Here are some things to look for in Ports.
Young Vintage Ports and other Ruby’s will be very dark purple while an older Vintage Port may be a very light ruby color or even tawny color.
Tawnies and Colheitas will often be more of a orange-amber-brown with red or golden highlights on the meniscus. Very old Colheita’s and Tawny’s will often have faint greenish glints on the edge.
Ports should be rather “clean” or clear in appearance. With the exception of really old Port or very young (under 3-5 years old), they should not have a cloudy appearance. If they do, this could simply be due to a poor decanting that took place in which there was sediment floating in the wine or it could be an indication of a flawed bottle.