I have an IPhone 5s and, at home, a cable modem. How fast are they?

I know the advertised internet speeds but what is the best, most accurate way to find their actual speeds?

By going by the Peak Times: Cable is Fast but is shared meaning many people are using your same connection…

Most residential broadband technologies, such as FTTX, Satellite Internet, or WiMAX, a population of users share the available bandwidth. Some technologies share only their core network, while some including Cable Internet and PON also share the access network. This arrangement allows the network operator to take advantage of statistical multiplexing, a bandwidth sharing technique which is employed to distribute bandwidth fairly, in order to provide an adequate level of service at an acceptable price. However, the operator has to monitor usage patterns and scale the network appropriately, to ensure that customers receive adequate service even during peak-usage times. If the network operator does not provide enough bandwidth for a particular neighborhood, the service can become sluggish if many people are using the service at the same time. Operators have been known to use a bandwidth cap, or other bandwidth throttling technique; users' download speed is limited during peak times, if they have downloaded a large amount of data that day.

Peak times is basically when everyone on your shared network is online at the same time.