Diagnosis - tell tale signs

For me they would be as follows:

Muscle pain on walking, particularly uphill, with first calves, then thighs, then chest and arms all aching until there is no alternative but to stop. A short rest revives and enables you to carry on.
A steady pace can be maintained for often long periods but any attempt to up the pace is likely to lead to the same symptoms as above.

Stairs cause thigh pain and it is simply not possible to go on at a certain stage because muscles become frozen.

Over exertion leads to severe muscle stiffness which can last days.

Simple every day actions cause muscle spasm- opening a jar often causes my hand to spasm.

Not being able to keep going when doing tasks- sawing would always see me stop well before what would be considered normal.

Finally if your urine is coloured by what you think is blood after exercise this is a sign of muscle breakdown (rhabdo)- I have had this happen 4 or 5 time each time after exercise and I knew it was somehow muscle related. If it happens to you make sure to ask for a CK (Creatine Kinase) blood test as elevated levels are easily detected and are a sign that McArdle's might be the problem rather than a kidney complaint.