BeltsBelts are easy to check with flow: there's a circuit condition that says "update every time a new item shows up." Building a flow meter is as simple as counting how many of these events occur in a given time period (say, a second). There are two ways of doing this, both of which are shown here:
The top contraption is a low-resolution flowmeter: it will update every 60 ticks with the amount transferred over the previous 60 tick interval. Its advantages are that it's more space-efficient, and you can multiplex different items on the same flowmeter and read them individually. It uses a clock to send a [GREEN] control pulse every 60th tick that sets and resets the signal.
Fluid flowmetersFluids are harder, because there's no pulse mode. However, there is a trick that makes it possible: the amount of fluid transferred in pipes is proportional to the difference in fluid height. So take two tanks, put them next to each other, and read the difference between their levels (that's a combinator of [EACH] * 1 for the upstream and [EACH] * -1 for the downstream tank). Because of fluid mechanics, it's difficult to accurately convert that to an exact fluid count, but it's possible to eyeball it to an approximate number--I divide the value by 45 to convert steam flow to approximate power consumption for my nuclear power plant. It's off by a little, but I just need enough to make sure I don't turn all my nuclear power plants if I'm using ⅓ the power. The other difficulty is that, because you're using tanks instead of pipes to read fluid height, it takes a long time for the values to quiesce when dramatic changes occur.
I'd really like it if there were a circuit network on pipes to measure fluid height or, better yet, amount_transferred.