You can tell which way round an actin filament is by labelling it with fragments of myosin molecules, which bind to it in a very particular way, and then looking at it really, really closely. People describe the appearance as arrowheads or chevrons, although to me it looks like ears of wheat:

http://jcb.rupress.org/content/jcb/79/3/846.full.pdf

From that, there is a convention of drawing actin filaments with chevrons, such as in this paper by my old supervisor:

https://www.semanticscholar.org/paper/Molecular-mechanism-of...