It isn't that clear. Those marking suddenly appear on Hashirama when he powered up. They also appeared on Tsunade when she activated her crystal, using the same powers as Hashirama to regenerate herself without hand seals. So does this mean Tsunade has sage mode when she activates her crystal or is it just a result of the regeneration technique?
Jiraya had to physically put similar symbols on himself when he entered sage mode against Pain. So this lends credibility as to it being a sign of sage mode. However, Naruto does not use these marking, nor do they appear when he enters sage mode, save for the tinting around his eyes. Just saying its not that obvious that its sage mode.