Well, first of all, ask the seller if the detergent appears to be hardened (if it isn't mentioned in the description). Look at the pictures to see if the box looks like it is bulging badly. Some bending is to be expected in something that old, but the box shouldn't look like it's about to burst.
The box shouldn't have water stains on it...some of the stuff looks like it's been in a flood.
Generally, concentrated detergents such as All, Dash, and AD (a short-lived Colgate brand) seem to have the most problems with hardening. I don't know whether this is because the surfactants were different or if the "fillers" in the un-concentrated brands help them to be freer-flowing.
If the detergent is from the phosphate-era, which is almost anything before 1970, I've had fairly good luck, even with All (though not so much with Dash!) After 1970 or so, some states restricted phosphorus content, but in much of the country, phosphate formulations were still on the market. If you can determine from pictures whether it has phosphates or not that helps. I've found that non-phosphate detergents get hardened much more readily, probably because they use sodium carbonate.
I've found dirty, faded boxes that are full of perfectly usable product, and minty-clean boxes that are hard as a rock.
Hope that helps!