So my client is asking to quickly tell him if a certain task is doable. But there is no way of just quickly checking its feasibility except for actually performing the entire task. How do I intimate this thing in a nice but solid manner? Is there a phrase or something that would convey my position?
The idiomatic expression, at least in the US, is "We won't know until we try."
In the software world, we'd say that we'd have to create a proof of concept.
"There's (only) one way to find out" with or without the word "only" kind of indicates "the best way to see what will happen is to try it"
An informal alternative to Hot Lick's contribution is
suck it and see UK informal - Cambridge Dictionary
to try something to find out if it will be successful: I'm not sure whether this paint is the right colour for the bedroom - we'll just have to suck it and see.
The phrase is in common use in Australian English. It might come across as off-colour on first presentation, but I think the imagery is derived from the taste-testing of confectioneries (e.g. lollipops): the only way to know how exactly it tastes is to sample it. I've highlighted the phrase in the usage instances quoted below.
Here's an instance of the phrase as a title in The Economist, a mainstream publication for the financial industry. The use of the phrase as a pun (in an article about a vacuum-cleaner entrepreneur) supports the assertion that the phrase is widely used.
- Suck it and see: The hazards of being an entrepreneur - The Economist
Here are a few more instances of the phrase in use in a positive light:
Pilots are an important outcome of the innovation process because they let an organization “suck it and see” before they take whatever-it-is to a broad base of customers. - James Gardner, Confusing the Pilot and the Prototype, Innovation Excellence
The visa should be OK, as long as they don't refuse us a couple of times like they did in Bangkok, but really we'll just have to suck it and see as they say. - The Angel Tree by Lucinda Riley
He wasn't sure yet whether he would like college refectory food, but he would have to suck it and see. Suck it and see? That was one of his mam's sayings, and one he had come to use a lot lately. - Jack by Frank English
And finally, here's the phrase used in a client brochure. It portrays the approach in a negative light and is further evidence of the phrase's acceptance in a commercial setting:
- The secret to getting the most out of a CDN campaign is to create content that is share-worthy. You can’t afford to take a suck-it-and-see approach. - Quantum Linx
One way of explaining the situation, is by saying that the question cannot be answered without first doing a pilot study / feasability study. In this case, the pilot study would obviously include virtually completing the project itself, but it might still be a way of explaining by using terms that are fairly well known.
If you want to be unambiguous, "You can only know [try, know if it will work] by trying it."