There are three main indicators of the strength of a trend:
- Movement in the direction of the underlying trend;
- Correction or consolidation before the primary trend resumes; and
- Support and resistance at the preceding high.
The magnitude of the move above the previous base indicates buyers' enthusiasm. The more bullish buyers are, the further they will be willing to venture above the safety net of the previous correction or consolidation.
Buyers below have thrown caution to the winds. Spikes like this often end with a sharp reversal as there is no clear support level.
Short corrections and narrow consolidations also reflect trend strength. A short correction indicates that buyers have overwhelmed sellers or simply that sellers are disinterested. In either case this is a positive sign for the underlying trend.
Narrow consolidations or rectangles are similarly a bullish continuation signal in an up-trend. Sellers may be present in reasonable force but buying support is sufficient to prevent a correction.
When price breaks above its preceding high, resistance at the earlier high becomes support for the new breakout, with buyers likely to accumulate should price retrace near the breakout level. The new support level will be relatively weak compared to support at the previous low; so when the new level holds it indicates a strong up-trend.
If initial support does not hold at the first attempt, buyers and sellers may be more evenly matched and the up-trend is likely to be slower and more uncertain. If price later respects the initial support level, that is a sign that trend strength may be increasing.
If the initial support level is breached and then acts as resistance on the next up-swing, the trend is precarious. Any penetration of key support (from the previous low) would complete a head and shoulders reversal, triggering a spate of selling.
Most traders will want to trade fast or reliable trends, where retracements respect the initial line of support. Slow up-trends can be traded in a similar fashion to a ranging market, using swing trading tactics, while I do not suggest that you trade weak trends at all.