What are some ways to measure market liquidity?

Other than "market depth" software, what is the best measure of liquidity (with an ECN broker) ?

Is spread the best measure?

Or perhaps some combination spread/volume is the best measurement?

Perhaps S/R levels are a key component as well? (I would assume liquidity is high here)

P.S I am talking about inter-day trading

Enter a large market order and see how far price moves?

Well, with inter-day trading it shouldn't really matter, you can just scale in over several minutes since the position's going for more than a day.

With intra-day trading, a volume indicator is the best, but if you're looking for something different those things you mentioned are all a good predictor of volume and pending orders.

With Forex, though, it shouldn't ever matter unless you're trading with triple-digit lot sizes, because volume is high throughout the day.

By the way, if you're trying to use areas of high volume to predict price movement, it doesn't work.

I hope I got my numbers right on this,
and I realize most people already know this stuff --

Liquidity may change at regular times,
according to (which I got here) :


overall
"Europe is responsible for nearly 53 percent of daily volume (the UK alone does almost one-third of daily global turnover)....Asia 21 percent"before Europe ".... North America 22 percent" after Europe.

sessions, daily trading
The article doesn't list liquidity each pair, except implicitly from common sense.
Per session, there's an Asian difference (Tiawan-Japan) and a European difference (France/Germany - London) that, last time I checked, adds a half or full hour to each market, changing at least the liquidity that much.
According to an investopedia article, an old (see http://www.cmegroup.com/trading_hours/fx-hours.html ) American difference where CME used to close at 3 pm and the US stock market does still at 4 pm EST, may also change liquidity.
The busiest overall is Europe, is 7 or 7:30 am to 3:30 pm or 4 GMT (highlights for all three countries listed).

slowest time of the day
10-12 pm GMT, when only the Australia session, the fourth session, is open. Maybe only 11-12 pm GMT in US earnings season.

weekend "gap"
(also from the first article)
"the early hours of the week’s trading tend to be stop-loss hunting expeditions. Such forays tend to occur following sharp moves into the close of the previous week’s trading. The result is frequently an opening flurry of action that sees nearby stop-loss levels triggered, only to see prices subsequently reverse once the stops are placed.

big news
Forex Factory red-flags these.
Of course, there's also the errant order flow of big nonmember countries' news or orders, which following rumors may pick up on, or broken-up pieces of big fills (indicator?).

a monthly change
From the first article:
"liquidity in USD/JPY tends to be thinnest on the last trading day of each month and at the end of financial semesters (March and September)"
Maybe other currencies too; not sure.

local stock market earnings season
In the US, especially, which happens after 4 pm EST and mostly begins with Alcoa (Intel, for tech, as I recall,) reporting.

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technicals
The first link, the article, also discusses technical behaviors of currencies in terms of liquidity.

current time -- A link for current GMT is here.
A forex sessions graph that is instantly adjustable each time zone is here.
And, each broke r has a different time zone for their MT4 platform.