You can consider a break of a long term trendline as valid only if the price visits that level again and close below it. Additional confirmation can be gained on shorter-term TF if the price works its way down to the trendline again and closes below it at any subsequent time.
I have already mentioned some things you can look at however, if you have a mind to look into tape reading then the trend line break is a reaction (pb) so in tape reading you are going to watch the volume on that reaction. Generally: 1) Decreasing volume on reactions (pd’s) favor a resumption of the previous trend (i.e. resumption of the previous bull trend) 2) Decreasing volume on rallies (pb) favor resumption of the decline (i.e. resumption of the previous bear trend) What you want to watch for is “dullness” on reactions in bull trends and “dullness” on rallies in bear trends. 1b) Said another way. When the volume increases on the advancement of the bull trend but decreases (light volume) on the PB’s then odds favor a resumption of the bull trend. 2b) Said another way. When volume increases on the advancement of the bear trend but is light on the pauses or PB’s then this sort of action favors a resumption of the bear trend. On charts tape reading is an interpretation of volume correlated with price movement as seen graphically on a chart. This type of tape reading IS NOT time and sales. The action of the volume renders hints about supply/demand. Price simply shows the monetary “value” of the volume. Volume shows the amount of dollars in play at price levels. If you don’t mind holding through drawdowns this type of tape reading can render nice profits. However, if a wrong interpretation is made it can result in devastating losses. I am a scalper of 1 to 8 points over and over in ES, MES, so I rarely look at volume. It just adds another consideration to making decisions that I find not necessary to scalping 1 to 8 points. The old timers used to say: “In a bull trend never sell a dull market” The same applies in a bear trend.