How do I know whether a potential investor is stalling vs. actually patiently evaluating the opportunity?

It's completely legitimate and polite to ask the ones doing diligence what their process and timeline is for a term sheet.

Unfortunately many VCs like to hang out and collect info and in the process essentially lead you on. Ultimately it's most important to have the "where do we stand" conversation. You'll be able to gauge who's serious about making a commitment.

It's perfectly fine to say “shit or get off the pot!” to clear out unserious inquiries. It's all about the delivery: at the start, ask for the VC's timeline and establish your own timeline so that you can say that if we don't get commitment/termsheet by X date, we will assume you're passing and we'll move on. By commitment, I mean a clear statement of "we're in for $XX", and maybe "we'll send termsheet by Xday" and you restate in writing by email and ask them to confirm in their reply (see PG/YC article on handshake protocol).

You should not have to actually be at end or raising process to confirm whether/not the interest was genuine.
You also don't want to fall prey to giving too much proprietary info out to unserious parties, who may then pass it on to your competitors that they pick over you.

Here's some advice based on dozens of closed rounds: Keep this vc as a stalking horse. Without a competing bid (btw, you should be out pitching and pitching until you get multiple term sheets), there's no urgency on the part of the vc to take any action. The vc will only move when he knows he's in danger of losing the opportunity. There are artful ways to pressure him; pressing him to declare his intentions is not one of them. You have no leverage in the current situation (so go get some).