Written instructions would usually mention right up if a pattern requires one-shuttle, one-shuttle-and-ball or two-shuttles. This may not be so obvious if looking at a chart of the design. So, my focus here will be on charts, specifically the style that I use for my patterns.
This chart is for a flower motif and is made entirely out if rings using one shuttle only.
The second chart is a shuttle-and-ball pattern. Notice that the rings are on the inside and all the chains are curved on the outside. However, the rings on their own are all facing the same way towards the centre, and each of the chains are curved in the same direction.
This third chart is also made with shuttle-and-ball, even though it looks different. Here, the rings are on the outside and the chains are in the centre. But, the point to note is that the rings are all facing to the outside and the curves in the chain are the same.
The same cannot be said for this chart. This is an example of a pattern that requires two shuttles.
I will go into a bit more detail using the chart below.
Another clue to look out for to know if you need one shuttle or two shuttle is to look at the curve in the chains. Using the chart above as an example, notice that the chain between the ring (A-A) or (B-B) is curved in one direction while the chain after the rings curves it the opposite direction, both within the same row or round. They are telling you that the part if the chain between the rings is made with shuttle 1, and you have to switch to shuttle 2 to make the chain after it.
While I am it, lets is look at another step in tatting that is not obvious in charts, Reverse Work. Written instructions usually have the notation RW or rw shown at the end of each step to indicate that you need to reverse your work before starting the next part.
Using this example, after making ring A, you need to reverse work to make ring B, followed by the chain. The change in the curve of the chain will also indicate that you need to reverse work.