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Hard question. Limping is a sign you don't play well, and I've witnessed some very inconsistent approaches from the serial limpers.

More important than what exactly is in his range preflop is how he plays the flop when he hits 2nd pair, top pair no kicker, or when he's got air. It seems like limpers think they're being tricky, but after a while observing them many have huge betting-pattern tells on the flop. Make an estimate preflop, but think really hard about how he plays that range when he catches a piece of the flop.