Personally I always check the digital pulse.....a raised pulse is my first alarm bell.
Obviously there are other symptoms, but the digital pulse is something I check daily and would use it as a first indicator for LGL in a horse/pony that seemed otherwise normal.....no hint of lameness, heat in feet etc etc.
They get footy over rough ground. Increased pulses and hot feet would follow. Be careful, they can go from being a little bit sore to full on can't move in only a few hours. Do not under any circumstances, cold hose the feet. this will constrict the blood vessels and do more damage.
My old mare has just had a mild attack. She went a bit pottery. You can get raised pulses but I don't know much else about it. We gave her bute,box rest on shavings bed and soaked hay.