Buoys and the tide.
When in open water, if you are not certain of the direction of a tidal stream, it may be indicated by observing a channel buoy.
If you are not close enough to the buoy to identify the direction of the tide directly, they tend to lean in the direction of the current as in the picture below.
Sailtrain.co.uk is free to use, but if you feel you would like to contribute to the running and development costs you can donate via Paypal: