It is a sad fact for investors and tipsters alike that, just as no siren sounds when stock markets peak, no bell rings at the bottom, either.

But in the opinion of Anthony Bolton, the celebrated Fidelity stockpicker, we do have a reasonable proxy for spotting the trough: the moment when the best-performing sector of the past few years begins to wane and the worst-performing sector starts to recover.

In this case, that means the point at which the stock market favours financials — or, more specifically, banks — over miners. “That’s the key switch,” Mr Bolton said this week.

That long-awaited crossover carries echoes of the top of the millennial bull market, whose zenith was signalled by investors’ abandonment of the “new economy” for the old — the clearest indication of which was the sudden outperformance of previously neglected tobacco stocks over telecoms.

In March 2000 it was right to have sold Vodafone and bought British American Tobacco, and it has been so ever since. Since then, the former has fallen by 58 per cent and the latter risen by 680 per cent — a near-eightfold advance.

So is there any evidence, this time round, that investors should now sell Rio Tinto and buy Royal Bank of Scotland — the reversal of the blue-chip trade that over the past two years has proved the route to riches? Unfortunately not. Richard Crossley, the closely followed technical analyst for NCB Stockbrokers — who correctly called the Vodafone-BAT trade at the time — notes that the trends of the past few years remain very much in place.

“There is no sign of any deterioration,” he says. In contrast, the post-millennial switch was preceded for several months by sectoral price and volume trends that to the likes of Mr Crossley were powerful portents of change.

That is a blow for all those who proposed that BHP Billiton’s $175 billion (£89 billion) bid for Rio was unambiguous top-of-the-market stuff for miners in the same way that Vodafone’s bid for Mannesmann was for telecoms. But it also once again raises the question of whether the strength and length of the mining bull market makes it an investment bubble every much as inflated as its predecessor in technology, media and telecoms (TMT) stocks.

Plotting the absolute performance of UK mining from 1999 and TMT from 1995, Citigroup finds that the returns from the former have dwarfed that of TMTs in the late 1990s. “Mining is now Everest to TMTs Mont Blanc,” it observes, noting that £100 invested in the sector in 1999 would be worth about £1,500 today.

Nor is there any hint of a loss of momentum. The mining sector is up by 20 per cent in the past two months alone and has outperformed the FTSE all-share index by 30 per cent since the start of this year. That comes on top of a 50 per cent outperformance in 2007.

That all lends weight to the continued existence of the so-called “supercycle” in commodities, an extended and accelerated period of abnormal demand caused by the industrialisation of India and China. Other evidence — other than the advance of oil to nearly $128 a barrel yesterday — comes in the sustained size of the natural resources sector relative to TMTs.

The latter, which previously accounted for 10 per cent of the UK stock market, swelled to 35 per cent at its peak, before falling back to 10 per cent within a couple of years. Although Citigroup points out that it has historically been difficult for a group of sectors to maintain a stock market weight of more than 25 per cent to 30 per cent, the US bank finds that natural resources — mining, oil, gas and chemicals — have crossed the upper end of that threshold within the past few weeks.

Cut another way, whereas there were only three TMT constituents in the top 10 of the FTSE 100 in 2000 — BT, Vodafone and Cable & Wireless — there are now seven natural resources stocks in that same grouping. In that sense, miners are more influential as a sector than telecoms ever were.

However, if there is one overarching reason to believe in the existence of a cycle over that of a bubble, it lies in valuations. In short, the surging share prices of miners have merely kept pace with the growth of their profits. The sector has traded on an average 12-month forward earnings multiple of 12 times since 2000, a level at which it remains.

In contrast, between 1995 and 2000, the rating of TMTs leapt from 15 times prospective earnings to nearly 50 times. All the same, there are reasonable grounds for near-term caution. Citigroup identified four occasions in the past five years when miners jumped by 20 per cent in the space of two months — and on average have fallen 6 per cent the following month.

That suggests that a brief bout of profit-taking could lie ahead. However, the sector still sits dead centre within the three trends that continue to drive the stock market, and which have pushed the FTSE 100 up nearly 900 points since March: investors’ preference for strong balance sheets, an exposure to commodity prices, and emerging markets — all of which the banking sector, with the exception of Standard Chartered and HSBC, conspicuously lacks.

That implies that the switch which Mr Bolton and the rest of the stock market seeks remains, for the time being, frustratingly in the dark.

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